Я забыл почтовый индекс и не смог отправить письмо.

Забыл is the past tense masculine singular form of the verb забыть (to forget).

In Russian, past tense verbs agree with the subject in gender and number:

• я забыл = I forgot (said by a man)
• я забыла = I forgot (said by a woman)
• мы забыли = we forgot

So if the speaker is male, я забыл is correct. If the speaker is female, it would be я забыла почтовый индекс и не смогла отправить письмо.

This is a very common and important question.

• мог = was able / could
• смог = managed to / succeeded in being able to

Here, не смог отправить письмо means was not able to send the letter in the sense of a completed situation with a result.

Why perfective смочь here? Because the sentence talks about a specific finished event: the person tried to send the letter, but the result was failure.

Compare:

• Я не мог отправить письмо.
  = I couldn’t send the letter / I was unable to send the letter.
  This can sound more like a general state or circumstance.

• Я не смог отправить письмо.
  = I wasn’t able to send the letter / I failed to send the letter.
  This emphasizes the outcome in that particular situation.

In this sentence, не смог is the more natural choice.

Because забыть is perfective, and the sentence refers to a completed act of forgetting.

• забывать = imperfective, process/habit/repeated action
• забыть = perfective, one completed event

Here the speaker means: I forgot the postal code at that time, in that situation.

Examples:

• Я всегда забываю имена.
  = I always forget names.
  (habit, repeated action)

• Я забыл его имя.
  = I forgot his name.
  (one completed event)

So я забыл почтовый индекс is the natural form for this context.

It is in the accusative case, but because почтовый индекс is an inanimate masculine noun phrase, its accusative form looks exactly like the nominative.

• Nominative: почтовый индекс
• Accusative: почтовый индекс

This happens with many inanimate masculine nouns in Russian.

For example:

• Я вижу стол.
  = I see a table.
  (стол looks the same in nominative and accusative)

Since забыть takes a direct object, почтовый индекс must be in the accusative, even though the form does not visibly change.

For the same basic reason: it is a direct object in the accusative case, and for many inanimate neuter nouns, the accusative looks the same as the nominative.

• Nominative: письмо
• Accusative: письмо

So in отправить письмо, the noun is in the accusative, but there is no visible ending change.

Russian does not have articles like a, an, or the.

So:

• почтовый индекс can mean a postal code or the postal code
• письмо can mean a letter or the letter

The exact meaning depends on context.

In this sentence, English might translate it as either:

• I forgot the postal code and couldn’t send the letter
• I forgot the postal code and couldn’t send a letter

Usually the surrounding context tells you which one is intended.

Yes. Почтовый индекс means postal code or zip code.

A few notes:

• индекс by itself can mean index, code, or other kinds of identifying number depending on context.
• почтовый индекс specifically means the mailing/postal code.

In everyday Russian, people may sometimes just say индекс if the context is clear.

Because смочь means to be able / to manage, and it is normally followed by an infinitive describing the action someone was or was not able to do.

So:

• смог отправить = managed to send / was able to send
• не смог отправить = didn’t manage to send / wasn’t able to send

Other examples:

• Я смог прийти. = I was able to come.
• Она не смогла ответить. = She couldn’t answer.
• Мы смогли найти дом. = We managed to find the house.

Because the sentence is about a single completed intended action: sending the letter.

• отправлять = imperfective, process/repetition
• отправить = perfective, one completed sending

Since the speaker is talking about whether the letter was successfully sent in that specific situation, Russian naturally uses the perfective infinitive:

• не смог отправить письмо

This means the action did not reach completion.

If you used отправлять, it would sound more like discussing the process or a repeated action, which does not fit as well here.

И simply means and. It connects the two parts:

• Я забыл почтовый индекс
• не смог отправить письмо

The idea is that the second part follows from the first: I forgot the postal code, and so I couldn’t send the letter.

Russian often uses plain и even when English might express a stronger cause-and-effect connection.

You could make the connection more explicit with words like:

• поэтому = therefore / so
• из-за этого = because of this

For example:

• Я забыл почтовый индекс, поэтому не смог отправить письмо.

But the original sentence with и is completely natural.

Yes, it can often be omitted in Russian if the meaning is clear from the verb form and context.

So both are possible:

• Я забыл почтовый индекс и не смог отправить письмо.
• Забыл почтовый индекс и не смог отправить письмо.

The version with я is a bit more explicit and neutral. Russian often drops subject pronouns when they are obvious, but keeping them is also very normal.

The given word order is very natural and neutral:

• Я забыл почтовый индекс и не смог отправить письмо.

Russian word order is flexible, but changing it usually changes emphasis, not the core meaning.

For example:

• Почтовый индекс я забыл и не смог отправить письмо.
  Emphasizes the postal code.

• Я не смог отправить письмо, потому что забыл почтовый индекс.
  Same basic meaning, but with a more explicit cause structure.

So the original sentence is a standard, straightforward way to say it.

A woman would say:

• Я забыла почтовый индекс и не смогла отправить письмо.

Both past-tense verbs change to the feminine form:

• забыл → забыла
• смог → смогла

That is because in Russian, past tense verbs agree with the speaker’s gender when the subject is я.

Not exactly.

• не отправил письмо = did not send the letter
• не смог отправить письмо = was not able to send the letter / couldn’t send the letter

The second version gives a reason of inability. It suggests that the person wanted or intended to send it, but something prevented it.

In this sentence, that reason is the forgotten postal code.

So:

• не отправил письмо focuses on the fact that the sending did not happen.
• не смог отправить письмо focuses on the fact that it was impossible or failed.