Я записал код подтверждения в блокнот, чтобы не забыть его.

Записал is the perfective past form of записать, so it presents the action as completed: the speaker wrote the code down successfully.

• Я записал код... = I wrote the code down
• Я записывал код... would sound more like I was writing down the code / I used to write down the code, focusing on the process or repeated action

In this sentence, the point is that the action was done once, as a completed step, so записал is the natural choice.

Yes. In the past tense, Russian verbs agree with gender and number.

• записал = masculine singular
• записала = feminine singular
• записало = neuter singular
• записали = plural

So Я записал... normally means the speaker is male. A female speaker would say Я записала код подтверждения в блокнот...

Yes, it can be omitted if the subject is already clear from context:

• Записал код подтверждения в блокнот, чтобы не забыть его.

However, in the past tense, the verb does not show person clearly the way present-tense forms do, so я often helps make the sentence explicit and natural, especially out of context.

So:

• Я записал... = clear and neutral
• Записал... = possible, but more dependent on context

A learner often notices that both can relate to writing, but they are not the same.

• записать usually means to write down / to note down / to record
• написать usually means to write something, often creating a text

Here, записал код подтверждения means the speaker quickly noted the code somewhere so they would have it later.

Compare:

• Я записал номер телефона. = I wrote down the phone number.
• Я написал письмо. = I wrote a letter.

So записал is exactly the right verb for writing down information for future use.

Подтверждения is in the genitive singular.

Russian often uses the pattern:

• noun + noun in the genitive

to express something like English X of Y or Y X.

So:

• код подтверждения = literally code of confirmation
• natural English equivalent: confirmation code

This is a very common structure in Russian:

• номер телефона = phone number
• чашка чая = cup of tea
• центр города = city center

This is about the case used after в.

• в + accusative often suggests direction or putting something into something
• в + prepositional usually gives location: in something

Here, записать в блокнот is the normal expression for write something down in/into a notebook.

So:

• в блокнот = into the notebook / down in the notebook
• в блокноте = in the notebook, describing location rather than the act of putting the information there

In other words, Russian treats this as an action directed into the notebook.

Because чтобы introduces a subordinate clause of purpose.

The sentence has two parts:

• Я записал код подтверждения в блокнот
• чтобы не забыть его

The second part explains why the speaker did the first action. In Russian, clauses introduced by чтобы are normally separated by a comma.

Here чтобы means so that / in order to.

It introduces the speaker’s purpose:

• Я записал код подтверждения в блокнот, чтобы не забыть его.
• I wrote the confirmation code down in a notebook so that I wouldn’t forget it.

It is one of the most common ways in Russian to express purpose.

Both are possible, but they are used a little differently.

чтобы не забыть

This is very common when the subject of both actions is the same.

• I wrote it down
• I did that so I would not forget it

Because the subject is the same, Russian often uses чтобы + infinitive.

чтобы я не забыл

This is also grammatically correct, but it sounds more explicit:

• Я записал код подтверждения в блокнот, чтобы я не забыл его.

It is usually less natural here unless you want extra emphasis. In everyday speech, чтобы не забыть его is the more idiomatic version.

Because the idea is to avoid one complete event of forgetting.

• забыть = to forget (perfective, a completed event)
• забывать = to be forgetting / to forget habitually or repeatedly (imperfective)

Here the speaker wrote the code down in order not to forget it at the needed moment. That is a single intended result, so забыть fits best.

Compare:

• чтобы не забыть его = so as not to forget it
• чтобы не забывать его = so as not to keep forgetting it / not to forget it repeatedly

The second one would sound odd in this context.

Его is the pronoun it/him, and here it means it, referring back to код.

So:

• код = the code
• его = it

The full phrase не забыть его means not to forget it.

In this sentence, его is the direct object of забыть.

Yes, sometimes it can be omitted if the meaning is obvious from context:

• Я записал код подтверждения в блокнот, чтобы не забыть.

That can still be understood as so I wouldn’t forget it.

But including его makes the sentence clearer and more complete, especially if the sentence is standing on its own. It clearly points back to код подтверждения.

Yes, Russian word order is flexible.

The version here is neutral and natural:

• ...чтобы не забыть его

You could also say:

• ...чтобы его не забыть

That puts a bit more focus on его.

Russian often places the most predictable information later, and the default order here is very natural. So the original sentence sounds straightforward and neutral.

You could, but the meaning changes slightly.

• блокнот = notebook / notepad, often small and used for jotting things down
• тетрадь = exercise book / notebook, often like a school notebook

Since the action is writing down a code quickly so as not to forget it, блокнот sounds especially natural. It gives the idea of a handy notebook used for notes.

Yes, it sounds natural.

It has:

• a normal perfective verb for a completed action: записал
• a common collocation: записать ... в блокнот
• a standard purpose clause: чтобы не забыть
• a clear pronoun referring back to the code: его

So it is a good, idiomatic everyday Russian sentence.