In short

Lesson 2 gave you the sentence with no verb in it: البَيْتُ كَبيرٌ, “the house is big”. Arabic puts two nouns side by side and lets “is” live in the gap between them. Move that sentence into the past and the gap has to be filled - nothing about البَيْتُ كَبيرٌ can be bent into “was”. The word that fills it is كانَ, and it is an ordinary past-tense verb with a very particular effect on the two nouns behind it.

كانَ costs you two things. First, its stem is unstable: كانَ but كُنْتُ. That is not an irregularity to memorise but a rule with one condition, and once you see it you can predict every cell. Second, it re-labels the sentence: the old مُبْتَدَأ keeps its ḍamma, and the old خَبَر swaps its ḍamma for a fatḥa - كانَ البَيْتُ كَبيرًا. You have met that exact machine before, in لَيْسَ (lesson 23). لَيْسَ and كانَ are two members of one family doing the same job in different tenses, so half of this lesson is already in your hands.

The rule

1. كانَ goes in front, and the sentence turns past. Take any verbless sentence and put كانَ before it. Nothing is deleted, nothing is reordered, and no extra word appears:

الجَوُّ جَميلٌ. -> كانَ الجَوُّ جَميلًا. / أَخي مُهَنْدِسٌ. -> كانَ أَخي مُهَنْدِسًا.

The English “was / were” is one word and so is كانَ, but the resemblance stops there: كانَ is conjugated like any past-tense verb of the type you drilled in lesson 18, with all ten person-endings, and because the sentence now opens with a verb most grammarians count it as a جُمْلَة فِعْلِيَّة (lesson 19). The noun behind it is not a فاعِل, though: كانَ names no action, and what follows it is the old مُبْتَدَأ with its own predicate still attached.

2. The two halves get new names. Once كانَ is there, مُبْتَدَأ and خَبَر are no longer the right labels. The first noun becomes اسْمُ كانَ, “the noun of كانَ”, and the second becomes خَبَرُ كانَ, “the predicate of كانَ”. The names matter because from this moment the two slots behave differently, and every later lesson about this family (43, 44, 59) uses them.

3. اسْمُها stays مَرْفوع، خَبَرُها becomes مَنْصوب. This is the whole grammatical content of the lesson. The subject keeps the ending it always had; the predicate changes:

Four sentences from your own stock, converted:

• البَيْتُ كَبيرٌ. -> كانَ البَيْتُ كَبيرًا.
• الطّالِبَةُ مَريضَةٌ. -> كانَتِ الطّالِبَةُ مَريضَةً.
• المُعَلِّمونَ مَشْهورونَ. -> كانَ المُعَلِّمونَ مَشْهورينَ.
• الطّالِباتُ مُجْتَهِداتٌ. -> كانَتِ الطّالِباتُ مُجْتَهِداتٍ.

Read the four right-hand columns aloud one after another. Only the second word of each pair moved; the first is untouched in all four.

4. “Accusative” is not always a fatḥa. مَنْصوب names a case, not a mark, and each kind of word shows it differently (the full system is lesson 40; here is the working slice). An indefinite masculine singular takes fatḥa-tanwīn on a supporting alif: كَبيرًا، مُتْعَبًا، مُهَنْدِسًا. A word ending in ة takes the two fatḥas on the ة itself with no alif: مَريضَةً، جَديدَةً، سَعيدَةً. The sound masculine plural does not use a fatḥa at all - it switches ـونَ to ـينَ: مَشْهورونَ -> مَشْهورينَ. The sound feminine plural takes a kasra: مُجْتَهِداتٌ -> مُجْتَهِداتٍ. The dual switches ـانِ to ـَيْنِ: سَعيدانِ -> سَعيدَيْنِ. And a definite predicate simply takes a bare fatḥa with no tanwīn: كانَ هٰذا الرَّجُلُ المُديرَ.

5. The non-human plural keeps its lesson-11 agreement. Putting a sentence into the past does not change who agrees with what. If the subject is a plural of things, the verb is feminine singular and the predicate is feminine singular too:

كانَتِ الكُتُبُ جَديدَةً. / كانَتِ الغُرَفُ نَظيفَةً.

Not *كانَتِ الكُتُبُ جَديداتٍ and not *كانوا الكُتُبُ. Two rules are stacked here and learners usually drop the older one: lesson 11 decides the shape of the predicate, this lesson decides its ending.

6. كانَ is أَجْوَف: the stem shortens to كُنْ. The root is ك-و-ن - you can see the و in the present يَكونُ and in the noun كَوْنٌ. In كانَ that و has stretched into the long alif. A long vowel does not survive in a closed syllable in the verb, so the moment you add an ending that begins with a consonant - and every such ending starts with a sukūn on the stem - the alif drops and leaves a short ḍamma behind:

كانَ + ـْتُ -> كُنْتُ ، كانَ + ـْنا -> كُنّا ، كانَ + ـْتُمْ -> كُنْتُمْ

Endings that begin with a vowel leave the alif alone: كانَ، كانَتْ، كانوا، كانا، كانَتا. So the paradigm splits cleanly in two, and *كانْتُ - alif plus sukūn - is not a shape an Arabic verb takes. (One extra detail: هُنَّ adds ـْنَ to كُنْ, and the two نs merge into a shadda: كُنَّ.) The shortening happens in every hollow verb, though the short vowel it leaves behind varies - قالَ gives قُلْتُ, باعَ gives بِعْتُ with a kasra - and lesson 50 turns it into a general rule. Here you only need this one verb.

7. Before a full noun, كانَ agrees in gender only. كانَ is a verb standing in front of its subject, so lesson 19 applies unchanged: it stays singular and matches gender:

كانَ الطُّلّابُ مُتْعَبينَ. / كانَتِ الطّالِباتُ مُتْعَباتٍ.

The plural forms كانوا and كُنَّ appear only when no subject noun follows, because then it is inside the verb itself: كانوا مُتْعَبينَ. And note the little kasra on كانَتِ: the ت of the feminine ending is vowelless, so before الـ it borrows a kasra to link the two words (lesson 18).

8. A شِبْه جُمْلَة predicate does not change at all. If the predicate is a preposition with its noun, or a word like عِنْدَ or هُناكَ (lesson 17), there is no single word that could take an accusative ending, and nothing happens except that كانَ appears:

الكِتابُ عَلَى الطّاوِلَةِ. -> كانَ الكِتابُ عَلَى الطّاوِلَةِ. عِنْدي كِتابٌ. -> كانَ عِنْدي كِتابٌ. هُناكَ اِجْتِماعٌ. -> كانَ هُناكَ اِجْتِماعٌ.

Look hard at the last two. كِتابٌ and اِجْتِماعٌ are not predicates - they are the delayed subject, اسْمُ كانَ, so they stay مَرْفوع. *كانَ عِنْدي كِتابًا is a very common error, produced by the reflex “كانَ means accusative” applied to the wrong word. The predicate is عِنْدي, and a preposition-phrase has no ending to give.

9. كُنْتُ أَدْرُسُ = “I used to study / I was studying”. The predicate can also be a whole sentence, and the most useful case is كانَ followed by a المُضارِع verb. That combination is how Arabic says the two things a bare الماضي cannot say: a habit in the past and an action in progress in the past.

كُنْتُ أَدْرُسُ العَرَبِيَّةَ كُلَّ مَساءٍ. - I used to study Arabic every evening. كانَتْ تَعْمَلُ في مَطْعَمٍ صَغيرٍ. - She was working / used to work in a small restaurant.

Both verbs carry the person, and they must carry the same one: كُنْتُ أَدْرُسُ، كُنّا نَدْرُسُ، كانوا يَدْرُسونَ. The مُضارِع keeps its ordinary مَرْفوع ending - there is no case to change, because a sentence has no ending. Use the pattern; do not go looking yet for its relatives.

10. لَيْسَ is the same machine in the present. لَيْسَ البَيْتُ كَبيرًا and كانَ البَيْتُ كَبيرًا are built identically: a conjugated verb-like word, an اسْم left مَرْفوع, a خَبَر pulled into the مَنْصوب. Even the conjugations rhyme, because لَيْسَ splits its stem exactly where كانَ does (لَيْسَ / لَسْتُ, كانَ / كُنْتُ). If you can do one, you can do the other; كانَ is simply the family’s head, and lesson 43 introduces the rest of it.

11. The ending you can actually hear. Case endings vanish at a pause - the ـٌ of كَبيرٌ and the ـٍ of مُجْتَهِداتٍ are simply not pronounced when you stop. The accusative ـًا is the exception: in الوَقْف the tanwīn softens but the alif stays, so كانَ البَيْتُ كَبيرًا really does end in an audible long ā. Say كُنْتُ مُتْعَبٌ to an Arabic speaker and the mistake is heard, not merely mis-spelled. This is worth drilling out loud for that reason alone.

12. Not in this lesson. كانَ has a large family and a large future, all of it deliberately postponed: أَصْبَحَ، صارَ، ظَلَّ، ما زالَ - أَخَوات كانَ - are lesson 43; the negative لَمْ يَكُنْ needs the jussive and waits for lessons 35 and 37; كانَ قَدْ فَعَلَ, the past perfect, is lesson 45. One preview is free, because you already have the future (lesson 24): سَيَكونُ does precisely what كانَ does, in the other direction - سَيَكونُ الجَوُّ جَميلًا غَدًا.

Forms & patterns

كانَ in the past - the ten singular and plural persons

The three dual cells are left out above only to keep the table short; they are built the same way: أَنْتُما كُنْتُما، هُما كانا (m.), هُما كانَتا (f.). The endings themselves are exactly the lesson-18 set - nothing new is being learned here except the stem.

Where the stem breaks

Converting a nominal sentence

How the مَنْصوب shows on the predicate

كانَ + المُضارِع - “used to / was doing”

Examples

Common mistakes

• كانَ البَيْتُ كَبيرٌ. -> كانَ البَيْتُ كَبيرًا. - The اسْم keeps its ḍamma, the خَبَر takes the fatḥa. Leaving the predicate مَرْفوع is by far the most frequent كانَ error.
• كانَ الطُّلّابُ مُجْتَهِدونَ. -> كانَ الطُّلّابُ مُجْتَهِدينَ. - The sound masculine plural shows the accusative as ـينَ. There is no fatḥa to write, but the case change is real.
• كانْتُ في البَيْتِ. -> كُنْتُ في البَيْتِ. - A verb stem does not keep a long alif in front of a sukūn. Every consonant-initial ending shortens the stem to كُنْ with a ḍamma.
• كانَتِ الكُتُبُ جَديداتٍ. -> كانَتِ الكُتُبُ جَديدَةً. - A plural of non-human things takes feminine singular agreement (lesson 11), and كانَ does not suspend that rule.
• كانَ عِنْدي كِتابًا. -> كانَ عِنْدي كِتابٌ. - Here عِنْدي is the predicate and كِتابٌ is the delayed اسْمُ كانَ, so it stays مَرْفوع. A شِبْه جُمْلَة predicate has no ending to give.
• كانَ الطّالِبَةُ مَريضَةً. -> كانَتِ الطّالِبَةُ مَريضَةً. - كانَ is a normal verb and must match the gender of the noun after it, with the linking kasra before الـ.
• كانوا الطُّلّابُ مُتْعَبينَ. -> كانَ الطُّلّابُ مُتْعَبينَ. - A verb placed before its noun subject stays singular (lesson 19). كانوا is for when no subject noun follows.
• كُنْتُ مُدَرِّسٌ في مَدْرَسَةٍ صَغيرَةٍ. -> كُنْتُ مُدَرِّسًا في مَدْرَسَةٍ صَغيرَةٍ. - The subject is inside كُنْتُ, so the noun after it can only be the خَبَر, and it is مَنْصوب.
• كُنْتُ دَرَسْتُ العَرَبِيَّةَ كُلَّ مَساءٍ. -> كُنْتُ أَدْرُسُ العَرَبِيَّةَ كُلَّ مَساءٍ. - “Used to” is كانَ plus the present. كانَ plus a past verb is a different tense and waits for lesson 45.
• كانَ الكِتابُ عَلَى الطّاوِلَةَ. -> كانَ الكِتابُ عَلَى الطّاوِلَةِ. - After a preposition the noun is مَجْرور, whatever else is happening in the sentence.

Tips

• Ask one question before you convert anything: is the predicate one word, or a phrase with a preposition? One word takes the accusative; a شِبْه جُمْلَة takes nothing at all.
• The shortcut for the ear: a nominal sentence in the present ends ḍamma-ḍamma, in the past ḍamma-fatḥa. البَيْتُ كَبيرٌ -> كانَ البَيْتُ كَبيرًا. Two ḍammas is your alarm that كانَ was ignored.
• Chant كانَ with its predicate attached, never bare: كُنْتُ مُتْعَبًا، كُنْتَ مُتْعَبًا، كُنْتِ مُتْعَبَةً، كانَ مُتْعَبًا، كانَتْ مُتْعَبَةً… The ـًا then travels with the form and never gets dropped.
• Sort the paradigm by the first letter of the ending rather than memorising ten items: vowel keeps كان, consonant gives كُنْ. That is two facts instead of ten.
• Go back to the sentences you wrote for lessons 2, 12 and 21 and put every one of them into the past. Converting your own material is worth more than any new example set.
• Say the accusative out loud. It is one of the few endings that survives a pause, so it is the cheapest way to make your Arabic sound cased rather than merely written that way.
• Park the relatives for now: أَصْبَحَ and ما زالَ (lesson 43), لَمْ يَكُنْ (lesson 35) and كانَ قَدْ فَعَلَ (lesson 45). One verb, one case rule, is a complete and useful skill on its own.