In short

Casual Japanese - タメ口 tameguchi - is not sloppy polite Japanese. It is a separate setting you choose from the relationship, not the topic: end sentences in the plain form, drop です and だ, ask questions with intonation alone, and squeeze long endings into contractions (〜ている -> 〜てる, 〜てしまう -> 〜ちゃう, 〜なければ -> 〜なきゃ). Learn to understand all of it now, and keep producing です／ます until the relationship clearly invites otherwise.

The rule

Step 1 - Pick the register from the relationship. Japanese marks every sentence for social distance, and the choice is made before you open your mouth.

• です／ます (丁寧体 teineitai) - anyone senior in age or rank, anyone at work outside your own team, strangers, shops, customers, teachers. Also the default with a peer you have just met.
• タメ口 (普通体 futsūtai) - close friends, classmates of the same year, family, and juniors you know well. “タメ” means “the same” - the same year, the same level.

The cost of an error is asymmetric, and this matters more than any form in this lesson. Using です／ます with a close friend sounds cold or sarcastic - a small, quickly repaired misfire. Using タメ口 with a teacher, a boss or a stranger reads as contempt, and it is not repaired quickly. In practice the senior person drops first: when your 先輩 senpai starts speaking plainly to you and says something like 敬語はいいよ (keigo wa ii yo, “no need to be formal”), you may follow. Until then, stay polite. A learner is never penalised for politeness.

Step 2 - Take です and だ off the end. Casual style uses the plain form (lesson 17) as the sentence-final form, not just the subordinate-clause form.

Verbs and い-adjectives simply lose the ます／です: 行きます -> 行く iku, 高いです -> 高い takai. Nouns and な-adjectives are the interesting case. The plain copula is だ, but in real speech it is usually just dropped: 学生です -> 学生。 gakusei, きれいです -> きれい。 kirei. Keeping だ (学生だ。) is not wrong, but it is assertive and reads as masculine; the bare noun is the neutral everyday form. Before よ and ね, だ usually comes back for nouns and な-adjectives: 学生だよ gakusei da yo, きれいだね kirei da ne. Leaving it out there (学生よ, きれいね) is common too, but traditionally sounds feminine.

Step 3 - Ask questions with intonation, not か. In casual speech you do not add か. You raise your pitch at the end: 行く？ (iku?) “you coming?”, もう食べた？ (mō tabeta?) “eaten yet?”. Attaching か to a plain form - 行くか - is not the casual question; it is blunt, confrontational and male-marked, so avoid producing it.

Step 4 - Use 〜の for explanations. The explanatory 〜んです of lesson 20 has two casual shapes, and they split by intonation:

• Rising 〜の？ asks the listener to explain the situation: どうしたの？ (Dō shita no?) “what’s up (what happened)?”
• Falling 〜の。 or 〜んだ。 gives an explanation: 明日、試験なんだ。 (Ashita, shiken nan da.) “the thing is, I’ve got an exam tomorrow.”

The two falling shapes are not interchangeable, and the split runs along gender. Rising 〜の？ as a question is unmarked - every speaker uses it. Falling 〜の。 as a statement is female-marked; the neutral statement is 〜んだ。 So 行くの？ “are you going?” is safe for anyone, while 行くの。 “I’m going, you see” reads as women’s speech where 行くんだ。 does not.

The な-insertion rule survives intact: a noun or な-adjective takes な before の／んだ - 学生なの, 静かなんだ - never ×学生だの.

Step 5 - Drop the particles that carry no work. Casual speech omits を, は and が whenever context makes the role obvious: ご飯（を）食べた？, これ（は）何？. The particles that encode a direction or a role you could not guess - に, で, へ, から, まで, と - normally stay (に does slip out of a bare destination question: どこ行く？). Dropping で in コンビニで買った turns the sentence into a puzzle, because コンビニ could then be the thing bought.

Step 6 - Contract. Contractions are phonology, not new grammar: a long ending gets squeezed, and you can always expand it back. Three families cover nearly all of them.

• い drops out of 〜ている and its relatives: 見ている -> 見てる, 読んでいる -> 読んでる, 持っていく -> 持ってく.
• ては／では collapse to ちゃ／じゃ: 見てはだめ -> 見ちゃだめ, これではだめ -> これじゃだめ. Separately, 〜てしまう squeezes through 〜ちまう down to 〜ちゃう (and 〜でしまう -> 〜じゃう). The two arrive at the same ちゃ／じゃ shape by different routes, so read the whole ending: ちゃ／じゃ standing before だめ／いけない is ては／では, while ちゃう／ちゃった is しまう.
• 〜ければ／〜れば shrink to 〜きゃ／〜りゃ: this is why なければ -> なきゃ and 見れば -> 見りゃ are the same rule, not two.

Step 7 - Recognise the casual sentence-final particles. Beyond ね and よ (lesson 11), casual speech adds じゃん (confirmation), 〜っけ (recalling a fact you ought to know), だろ, かな, さ, and the heavily marked ぜ／ぞ／わ. These are received far more often than they should be produced. Take them in as comprehension; leave the marked ones in fiction where you found them.

Forms & patterns

Polite versus casual, cell by cell. Same sentence, two settings.

The contraction table. Learn to expand the right column back into the left.

〜なきゃ and 〜なくちゃ stand alone. The ならない／いけません of lesson 35 is simply left off: もう行かなきゃ。 (Mō ikanakya.) is a complete sentence, “I’ve got to go.”

Casual sentence-final particles.

Examples

Common mistakes

• 先生、今何してる？ (Sensei, ima nani shiteru?) -> 先生、今何をしていますか。 (Sensei, ima nani o shite imasu ka.) - the register follows the person, not how friendly the topic feels. A teacher, a boss or a stranger gets です／ます even for small talk.
• 行くか？ (Iku ka?) -> 行く？ (Iku?) - a casual question is marked by rising intonation alone. Adding か to a plain form is not casual-polite; it is blunt and confrontational.
• 学生だの？ (Gakusei da no?) -> 学生なの？ (Gakusei na no?) - before の and んだ a noun or な-adjective takes な, exactly as it takes な before んです. だ never survives there.
• コンビニ買った？ (Konbini katta?) -> コンビニで買った？ (Konbini de katta?) - を, は and が drop freely, but に and で carry the role of the noun. Drop で and コンビニ looks like the thing you bought.
• 昨日、映画を見てたです。 (Kinō, eiga o miteta desu.) -> 昨日、映画を見ていました。 (Kinō, eiga o mite imashita.) - you cannot bolt です onto a plain past verb; ×見たです is wrong for the same reason. Contracting is not what breaks it: 〜てる inflects like any 一段 verb, so the polite contraction 見てました is ordinary spoken Japanese. What is impossible is 〜てた + です.

Tips

• Comprehend everything, produce a narrow band. Native casual speech is contracted to the bone, so you must decode 〜てる, 〜ちゃう and 〜なきゃ instantly to follow a conversation or a drama. Your own output can stay in です／ます for a long time without ever sounding wrong. That asymmetry is the whole strategy for this lesson.
• Expand, don’t memorise. Every contraction unwinds into something you already know: 食べちゃった = 食べてしまった, 行かなきゃ = 行かなければ（ならない）, 待ってて = 待っていて. When an unfamiliar ending appears, run it backwards through the table instead of treating it as new grammar.
• Beware 役割語 (yakuwarigo, “role language”). Anime and manga hand out ぜ, ぞ, わ, あたし and わし to signal character type, not to record how people talk. Copying them makes a learner sound like a cartoon. かな and ね are safe everywhere; the rest are best left as recognition.
• A brief drop to plain form is meaningful, not a mistake. Even in a polite conversation, speakers slip into the plain form for a muttered aside or a thought spoken aloud - あ、雨だ (A, ame da, “oh, it’s raining”). This is 独り言 hitorigoto, talking to yourself, and it does not change the register of the conversation.