Я записал правило в дневник, чтобы не забыть его.

I wrote the rule down in my diary so that I wouldn’t forget it.

Breakdown of Я записал правило в дневник, чтобы не забыть его.

• я: I
• в: in
• не: not
• чтобы: so that
• забыть: to forget
• записать: to write down
• его: it
• правило: rule
• дневник: diary

Questions & Answers about Я записал правило в дневник, чтобы не забыть его.

Why is it записал and not записывал?

Записал is the perfective past tense, which presents the action as completed: I wrote it down (and it’s done).
Записывал is imperfective and would emphasize the process, repetition, or an unfinished/ongoing context: I was writing it down / I used to write it down.
In this sentence, the point is that the rule is now in the diary, so perfective записал fits.

What does the ending in записал tell me (tense/person/gender)?

Записал is:

• Past tense (Russian past is marked on the verb)
• Masculine singular (because the speaker is presumably male; a female speaker would say записала)
• The person (I) is shown by the subject я, not by the verb ending (past tense verbs don’t conjugate for person the way present-tense verbs do).

Why is правило in that form, and what case is it?

Правило is the direct object of записал, so it’s in the accusative case.
For neuter nouns like правило, nominative and accusative are often identical: правило (Nom.) = правило (Acc.).

Why is it в дневник and not в дневнике?

With в, Russian uses:

• Accusative (в дневник) to mean movement/putting into: into the diary / in my diary (as an entry)
• Prepositional (в дневнике) to mean location: in the diary (already there)

Here, you’re placing/writing something into the diary, so в + accusative is used: в дневник.

Does в дневник literally mean “into a diary”? How natural is that in Russian?

Yes—literally it’s “into the diary,” but idiomatically it’s exactly how Russian often describes making an entry.
Записать в дневник is a common collocation meaning to write down in one’s diary/journal.

Why is there a comma before чтобы?

Because чтобы не забыть его is a purpose clause (a subordinate clause). In Russian, subordinate clauses are normally separated by a comma from the main clause:

• Я записал правило в дневник, чтобы…

How does чтобы + infinitive work here?

Чтобы commonly introduces a purpose: “in order to / so that (someone) can”.
When the subject is the same as in the main clause, Russian often uses чтобы + infinitive:

• Я записал …, чтобы не забыть … = I wrote it down so as not to forget…

If you need a different subject, you can add it:

• …, чтобы он не забыл = so that he wouldn’t forget

Why is it не забыть (perfective) and not не забывать (imperfective)?

Забыть (perfective) points to a single potential event of forgetting: so that I don’t end up forgetting it.
Забывать (imperfective) would suggest a more general/habitual idea: so that I don’t forget it (as a general tendency).
In purpose clauses like this, perfective is very common when you mean preventing a one-time mistake.

Why do we need его at the end? What case is it?

Его is the pronoun “it/him” in genitive/accusative form. Here it’s accusative because it’s the direct object of забыть (to forget what?).
It refers back to правило (a neuter noun), and Russian uses его for neuter accusative objects.

It can sometimes be omitted if context is crystal clear, but including it is normal and often preferred for clarity:

• …, чтобы не забыть (его)

Is the word order fixed? Could I move parts around?

Russian word order is fairly flexible, but changes can shift emphasis. Common variants include:

• Я записал правило в дневник, чтобы его не забыть. (slightly more emphasis on it)
• Правило я записал в дневник, чтобы не забыть его. (emphasizes the rule as the topic)

The original word order is neutral and very natural.