In short

Both から (kara) and ので (node) mean “because”, and both put the reason first: reason + から／ので + result. から presents the reason as your own judgement - assertive, the natural choice when you push or invite, and the only option when you answer “why?”; ので presents it as circumstance beyond dispute - softer, and the safe default in requests, apologies and excuses. The one form to memorise is な before ので after a noun or な-adjective: 雨なので ame na node, never 雨だので.

The rule

Step 1 - Reason first, result last

Japanese puts the subordinate clause before the main clause, so the reason clause comes first and the sentence still ends with the main verb:

[reason] + から／ので、[result]。

• 時間がありませんから、タクシーで行きます。 (Jikan ga arimasen kara, takushī de ikimasu.) - “There’s no time, so I’ll go by taxi.”

English can start with the result (“I’ll go by taxi because…”); the neutral Japanese order is the reverse. A comma (、) after から／ので is normal but not compulsory.

Step 2 - What から attaches to

から attaches to a complete predicate, either plain or polite - it is one of the very few connectives that tolerates です／ます in mid-sentence:

• 高いから takai kara / 高いですから takai desu kara
• 行くから iku kara / 行きますから ikimasu kara

After a noun or a な-adjective you need a copula: plain だ or polite です. 学生だから gakusei da kara, 学生ですから gakusei desu kara - never a bare 学生から (that would be the particle から “from”, lesson 6).

Step 3 - What ので attaches to, and the な trap

ので attaches to the plain form - strictly, the attributive (連体形) shape, the one that stands in front of a noun (see Relative (noun-modifying) clauses) - because ので is historically the nominalising の + で:

• 高いので takai node, 行くので iku node, 遅れたので okureta node, 分からないので wakaranai node

And exactly because that slot wants the attributive shape, a noun or な-adjective takes な - な is the attributive form of だ, the form that appears before の, ので and のに - and not だ itself:

• 雨なので ame na node, 静かなので shizuka na node, 日曜日なので nichiyōbi na node

Past tense is a plain past, with no な: 雨だったので ame datta node.

The one exception to “plain form only” is the stiff formal 〜ですので／〜ますので of Step 4 (雨ですので, 閉まりますので): です and ます have a restricted attributive form that surfaces in exactly this position, before ので／のに, and the pattern is firmly established in announcements and business speech.

Step 4 - から is subjective, ので is objective

This is the whole difference, and it is pragmatic, not grammatical.

から announces “this is my reason - I stand behind it.” It is fine for insisting, arguing, pushing, warning and ordering. That same assertiveness is why から can sound defensive when you are apologising: it reads as “look, it wasn’t my fault.”

ので presents the reason as a fact of the world that you are merely reporting, so no one has to accept your judgement. It is therefore the standard clause in requests, apologies and excuses, and in anything addressed to a customer, a teacher or a boss:

• すみません、電車が遅れたので、遅くなりました。 (Sumimasen, densha ga okureta node, osoku narimashita.)

Politeness stacks on top of this: 高いから (casual) < 高いので (everyday polite) < 高いですから (polite and emphatic) < 雨ですので／閉まりますので (formal announcements and business). Note the direction - 〜ますので is more formal than 〜ますから, not less.

Step 5 - Where から works and ので does not

Because ので disclaims subjectivity, it clashes with a main clause that is pure speaker will:

• Invitations 〜ましょう and volitional forms: から is the natural choice - 時間がありませんから、急ぎましょう。 ○ ので is not ungrammatical here (public notices really do say 危ないので、気をつけましょう), but in conversation 寒いので、窓を閉めましょう sounds oddly detached, as if you were reading out an announcement.
• Plain commands and prohibitions (lesson 30): 危ないから、触るな。 ○ - ので here is wrong.
• Answering どうして／なぜ: only から can be made into a predicate with 〜からです. 〜のでです does not exist.

Polite requests are the exception you will hear everywhere: 〜ので、〜てください is completely natural, and is in fact the house style of station and shop notices, precisely because it blames the circumstances rather than the speaker.

Step 6 - から as an afterthought, and ので trailing off

Reversing the order is possible with から: state the result, then add the reason as a separate sentence, as a justification tacked on.

• 早く帰りましょう。明日は朝が早いですから。 (Hayaku kaerimashō. Ashita wa asa ga hayai desu kara.)

ので does the opposite: it trails off with the main clause left unsaid, which is a very polite way to decline or excuse yourself - ちょっと用事があるので…… (Chotto yōji ga aru node……) “I’m afraid I have something on, so…”.

Step 7 - With んです, and the formal alternative ため

The explanatory 〜んです (lesson 20) supplies background rather than a cause, and the usual polite opener for a request is 〜んですが、…… - “the situation is X, and…” - leaving the request to follow: 道が分からないんですが、教えてください。 Use it when you want to set a scene; use ので when you want to state a cause.

だから and ですから (“so, therefore”) are the same から standing at the head of a new sentence; they belong with the connectives in lesson 22. In writing, notices and news, the formal reason marker is 〜ため（に）: 大雨のため、電車が止まりました。 (Ōame no tame, densha ga tomarimashita.) Recognise it now; the same word marks purpose in lesson 43.

Forms & patterns

Attachment - the だ／な split is the thing to memorise:

What the main clause allows:

Register ladder - same reason, four levels:

Examples

Common mistakes

• 雨だので、行きません。 (Ame da node, ikimasen.) -> 雨なので、行きません。 (Ame na node, ikimasen.) - after a noun or a な-adjective, ので takes な. Only から takes だ: 雨だから.
• 高いなので、買いません。 (Takai na node, kaimasen.) -> 高いので、買いません。 (Takai node, kaimasen.) - that な belongs to nouns and な-adjectives only; an い-adjective attaches to ので directly.
• 寒いので、窓を閉めましょう。 (Samui node, mado o shimemashō.) -> 寒いから、窓を閉めましょう。 (Samui kara, mado o shimemashō.) - not ungrammatical, but an invitation is the speaker’s own decision, so conversational Japanese reaches for the subjective から; ので makes it sound like a public announcement.
• 熱があったのでです。 (Netsu ga atta node desu.) -> 熱があったからです。 (Netsu ga atta kara desu.) - only から can be turned into a predicate to answer どうして; のでです does not exist.
• 道が混んでいましたから、遅れました。 (Michi ga konde imashita kara, okuremashita.) -> 道が混んでいたので、遅れました。 (Michi ga konde ita node, okuremashita.) - grammatical, but in an apology から sounds like you are insisting it was not your fault; ので just states the circumstance.

Tips

• The one-line mnemonic: から = “because I say so” (my judgement, my push); ので = “because that’s how things are” (circumstance, no argument). Facing a stranger, a customer or a boss - especially when apologising - reach for ので.
• から is a rare licence to be polite mid-sentence. 高いですから is correct, and more polite than 高いから. Do not generalise it: most subordinate clauses demand the plain form (×食べますとき -> ○食べるとき), and ので is one of them in ordinary speech.
• In casual speech ので contracts to んで: 高いんで takai n de, 遅れたんで okureta n de. It is everyday among friends, but it strips out exactly the deference that made you choose ので - never use it with a superior.
• Don’t confuse the two から. Particle から after a noun means “from” (九時から kuji kara, “from nine o’clock”); conjunction から after a predicate means “because”. The word before it tells you which.