Вчера я забыл дневник дома, поэтому не записал новое правило.

Yesterday I forgot my diary at home, so I didn’t write down the new rule.

Breakdown of Вчера я забыл дневник дома, поэтому не записал новое правило.

• я: I
• вчера: yesterday
• новый: new
• дома: at home
• не: not
• правило: the rule
• поэтому: therefore
• забыть: to forget
• записать: to write down
• дневник: diary

Questions & Answers about Вчера я забыл дневник дома, поэтому не записал новое правило.

Why is it забыл and not забывал?

Забыл is perfective past: it presents the forgetting as a completed one-time event (I forgot).
Забывал is imperfective past and usually means a repeated/habitual pattern (I used to forget / would forget) or focuses on the process/background rather than the single result.

Why is it не записал and not не записывал?

Не записал (perfective) means you didn’t do it (the result didn’t happen): you failed to write it down this time.
Не записывал (imperfective) often means you weren’t writing it down / didn’t used to write it down, or it can sound like background description rather than a single missed action.

How does past tense work in Russian—why забыл and записал?

Russian past tense agrees in gender (singular):

• masculine: забыл, записал
• feminine: забыла, записала
• neuter: забыло, записало
• plural: забыли, записали
  So забыл / записал tells you the speaker is (grammatically) male or the subject is masculine.

Why is there я? Can Russian drop I?

Yes, Russian often omits subject pronouns because the verb form already gives a lot of information (especially in present tense). In the past tense, the verb shows gender/number but not person, so я is still common for clarity.
You can say: Вчера забыл дневник дома, поэтому не записал новое правило. It sounds natural in context.

What case is дневник in here? It looks like nominative.

It’s accusative, because it’s the direct object of забыл (forgot what?).
But for inanimate masculine nouns, the accusative usually equals the nominative:

• nominative: дневник
• accusative (inanimate): дневник
  So it looks unchanged, but the role is accusative.

Why is новое правило not changing? What case is it?

It’s also accusative, as the direct object of записал (wrote down what?).
For inanimate neuter nouns, nominative and accusative are the same in the singular:

• nominative/accusative: правило
  And the adjective matches: новое (neuter singular).

Why is it дома and not в доме?

Дома is an adverb meaning at home / at the house (one’s own place) and is very common in everyday Russian.
В доме literally means in the house/building and is more physical/specific (inside the building), not necessarily “my home.”

Does дневник mean “diary” or “notebook”?

Дневник can mean different things depending on context:

• a school diary/record book (very common): where homework/notes/grades go
• a personal diary/journal
  In a sentence about a new rule and writing it down, it often implies a school notebook/diary rather than a private journal.

What’s the difference between поэтому and потому что?

• потому что = because (gives the reason): Я не записал правило, потому что забыл дневник.
• поэтому = therefore/so (gives the result/consequence): Я забыл дневник, поэтому не записал правило.
  They often pair as reason → result, but you typically choose one structure or the other.

Why is there a comma before поэтому?

Because the sentence has two clauses:
1) Вчера я забыл дневник дома
2) поэтому не записал новое правило
Russian uses a comma to separate such clauses, especially when the second is introduced by a connector like поэтому.

Is the word order flexible here? Could I move вчера or дома?

Yes, Russian word order is flexible and depends on emphasis. For example:

• Я вчера забыл дневник дома... (neutral, just moving вчера)
• Вчера я забыл дневник дома... (emphasis on “yesterday”)
• Вчера я забыл дневник дома vs Вчера я дома забыл дневник (the second can emphasize “at home” as the place where the forgetting happened).
  The given order is very natural.

Why isn’t there a word for “my” (as in “my diary”)?

Russian often omits possessives when ownership is obvious from context.
If you want to be explicit, you can say мой дневник:
Вчера я забыл мой дневник дома...
But the version without мой sounds more natural in many everyday situations.

What does the prefix за- in записал add?

Записать means to write down / record (as a completed action). The prefix за- helps form a verb with the sense of making a record / putting something into writing.
Related forms:

• imperfective: записывать (to be writing down / to write down habitually)
• perfective: записать (to write down once, successfully)