Learn Hardcore Russian: Я записал номер в тетрадь.

писать (imperfective) = to write (in general, be writing)
- я писал номер – I was writing the number / I used to write the number (process or repeated action, not a finished single result).
написать (perfective) = to write (and finish some text)
- я написал номер – usually sounds like I wrote the number (somewhere) but with more focus on the act of writing itself, not on storing/recording it.
записать (perfective) = to write something down, to note it, to record it
- Emphasizes recording for later use (not just writing shapes).

In Я записал номер в тетрадь, the speaker is saying they wrote the number down (to keep it), which is exactly the nuance of записать.

Why is it записал and not something like записалИ or записало?

Записал is masculine singular past tense.

Russian past tense endings agree with the subject’s gender and number:

masculine singular: записал (я записал, он записал)
feminine singular: записала (я записала, она записала)
neuter singular: записало (оно записало) – rarely used with people
plural (any gender mix): записали (мы записали, они записали)

So:

A man would say: Я записал номер в тетрадь.
A woman would say: Я записала номер в тетрадь.

Why does номер look like the basic dictionary form even though it’s an object?

Номер is a masculine inanimate noun.

For masculine inanimate nouns in the singular:

Nominative = Accusative (they look the same).

So:

Nominative: номер – the number (subject)
Accusative: номер – the number (object)

In Я записал номер в тетрадь, номер is the direct object in the accusative case, but its form happens to be identical to the nominative.

Why is it в тетрадь, not в тетради?

The preposition в can take:

Accusative = motion into / direction
Prepositional = location in/inside (where something is)

Compare:

в тетрадь (accusative) – into the notebook
→ indicates movement into: you are putting something into the notebook (writing it there).
в тетради (prepositional) – in the notebook
→ describes where something already is: номер в тетради – the number is in the notebook.

In Я записал номер в тетрадь, the idea is I wrote the number *into the notebook, so *accusative (в тетрадь) is used.

Why doesn’t тетрадь change form between nominative and accusative here?

Тетрадь is a feminine inanimate noun ending in a soft sign (-ь).

For feminine inanimate nouns:

Nominative singular: тетрадь
Accusative singular: тетрадь (same as nominative)

For feminine animate nouns, the accusative would look like the genitive, but a notebook is inanimate.

So в тетрадь uses the accusative case, but its form is identical to the nominative.

Is тетрадь masculine or feminine? How do I know?

Тетрадь is feminine.

Clues:

Many nouns ending in -ь can be either masculine or feminine, so you must learn them.
In dictionaries, you’ll usually see тетрадь, ж.р. (ж.р. = feminine gender).
Adjectives and past verbs would agree in feminine:
- новая тетрадь – a new notebook (новая = feminine)
- тетрадь была на столе – the notebook was on the table (была = feminine past).

Could I say Я писал номер в тетрадь instead? What’s the difference?

You can, but the meaning changes:

Я записал номер в тетрадь.
→ completed, single action, result is important: the number is now recorded.
Я писал номер в тетрадь.
→ focuses on the process: I was writing the number in the notebook / I spent some time writing it.
It doesn’t necessarily say that you finished or that the result is important.

In everyday speech, if you want to say I wrote it down, записал (perfective) is the natural choice.

What’s the difference between записать номер and запомнить номер?

записать номер – to write the number down (somewhere)
→ physical recording on paper / phone / notebook.
запомнить номер – to memorize the number
→ mental action, you remember it without needing notes.

So:

Я записал номер в тетрадь. – I put it into my notebook.
Я запомнил номер. – I learned it by heart.

Could I say Я написал номер в тетрадь instead of записал?

You can, and people will understand you, but the nuance shifts a bit:

написать номер в тетрадь – to write the number in the notebook
Neutral about why you wrote it; focus is on the act of writing.
записать номер в тетрадь – to write down / note down the number
Stronger nuance of recording information for later.

In most “I wrote it down so I won’t forget” contexts, записать feels more precise and natural.

Can I change the word order, like Я в тетрадь записал номер?

Yes. Russian word order is more flexible than English, though the neutral order is:

Я записал номер в тетрадь. (subject – verb – object – destination)

Possible variants:

Я в тетрадь записал номер. – slightly emphasizes в тетрадь (where you recorded it).
Номер я записал в тетрадь. – emphasizes номер (this number, not something else).

All are grammatically correct; the differences are mainly in emphasis and style. For a basic learner sentence, keep the original order.

Why is there no word for “a’’ or “the’’ before номер and тетрадь?

Russian doesn’t use articles (no “a/an” or “the”).

Definiteness/indefiniteness (“a number” vs “the number”) is determined by:

context
word order
sometimes other words (e.g. этот номер – this number).

So Я записал номер в тетрадь could be:

I wrote a number in a notebook.
I wrote the number in the notebook.

Which one is correct depends on the wider context, not on any special word form.

Could I drop я and just say Записал номер в тетрадь?

Yes, in context, that’s natural.

Russian often omits subject pronouns when:

the subject is clear from context and
the verb ending shows who did the action.

Записал номер в тетрадь would usually be understood as:

“(I) wrote the number in the notebook,” if you’re talking about yourself and the form is masculine, or
“(he) wrote the number in the notebook,” depending on the situation.

In careful, textbook-style sentences, learners are usually taught to keep я, but native speakers often omit it in conversation.

How is everything in this sentence stressed and pronounced?

Stresses:

Я – [ya], unstressed, very short.
записал – за-пи-са́л (stress on the last syllable)
номер – но́-мер (stress on the first syllable)
в – just [v], usually attached to next word in speech.
тетрадь – те́-традь (stress on the first syllable)

So the rhythm is roughly: я записа́л но́мер в те́традь.