Я чуть не забыла зачётку дома и поэтому вернулась за ней.

Чуть не + past tense verb means almost did something, but in the end it did not happen.

So:

Я чуть не забыла зачётку дома = I almost forgot my grade book at home.

Even though забыла is a past-tense form, the whole expression means the action was very close to happening, but was avoided.

A very literal way to think of it is: I nearly forgot it at home.

Because the speaker is female.

In Russian past tense, singular verbs agree with gender:

• забыл = a man forgot
• забыла = a woman forgot
• забыло = it forgot
• забыли = they forgot / polite you forgot

The same thing happens later in the sentence:

• вернулся = a man returned
• вернулась = a woman returned

So this sentence is spoken by a woman.

Because it is the direct object of забыла.

The dictionary form is зачётка. In this sentence it changes to the accusative singular:

• зачётка → nominative
• зачётку → accusative

This is the normal pattern for many feminine nouns ending in -а:

• книга → книгу
• машина → машину
• зачётка → зачётку

So забыла зачётку means forgot the grade book.

Зачётка is a colloquial short form of зачётная книжка.

It refers to a student record book used in Russian-speaking universities, where grades, passes, and exam results are recorded.

So in English, depending on context, you might translate it as:

• grade book
• student record book
• sometimes just leave it as record book

The shortened form зачётка is very common in everyday speech.

Here дома means at home.

So:

• забыла зачётку дома = left / forgot the grade book at home

This is different from домой, which means to home / homeward:

• Я пошла домой = I went home
• Я оставила книгу дома = I left the book at home

So in this sentence, дома tells you the location where the grade book was nearly left behind.

Поэтому means therefore, so, or that’s why.

In this sentence:

...и поэтому вернулась за ней = ...and therefore/so I went back for it

It connects the first idea to the result:

• I almost left it at home,
• so I went back for it.

Russian often uses поэтому in the middle of a sentence like this, but it can move depending on emphasis.

For example:

• Я чуть не забыла зачётку дома, поэтому вернулась за ней.
• Я чуть не забыла зачётку дома и поэтому вернулась за ней.

Both are natural.

Literally, it means something like:

returned for it

The verb is вернуться = to return / come back.

With за + instrumental, it can mean to come back for something:

• вернуться за книгой = come back for the book
• пойти за ключами = go get the keys
• вернулась за ней = came back for it

So the sentence means the speaker realized the problem and went back to get the grade book.

Because the verb pattern here is вернуться за + instrumental.

Russian often uses specific prepositions with specific cases. In the meaning go back for / come back to get, the preposition за takes the instrumental case.

So:

• зачётка → instrumental singular зачёткой
• pronoun она → instrumental singular ней

That is why you get:

• вернулась за зачёткой
• вернулась за ней

If you used её, that would be accusative/genitive, and it would not fit this construction.

Because it refers back to зачётку, and зачётка is a feminine noun.

Russian pronouns agree with the noun they replace:

• он for masculine nouns
• она for feminine nouns
• оно for neuter nouns

Since зачётка is feminine, the pronoun is feminine too:

• nominative: она
• accusative: её
• instrumental after за: ней

So за ней means for it, where it refers to the feminine noun зачётка.

Because the sentence presents two whole events:

• чуть не забыла = almost forgot
• вернулась = went back / returned

Perfective verbs are commonly used for single completed events or events viewed as a whole.

Here:

• забыла is perfective because it refers to the near-completion of a single act of forgetting/leaving it behind.
• вернулась is perfective because the speaker actually did return; it is one completed action.

If you used imperfective forms, the meaning would change or sound less natural in this context.

Yes, very often.

Russian does not always need an explicit subject pronoun when the verb form already makes the subject clear from context.

So a speaker could naturally say:

Чуть не забыла зачётку дома и поэтому вернулась за ней.

Because забыла and вернулась already show:

• first person singular
• feminine speaker

Russian includes Я when it wants extra clarity, contrast, or emphasis, but it is not always necessary.

Yes, there is a slight difference.

• забыла дома means forgot it at home / nearly left it at home
• оставила дома means left it at home

Забыть focuses on forgetting. Оставить focuses on leaving something somewhere.

In this sentence, чуть не забыла зачётку дома suggests the speaker almost went out without it, then realized it and turned back. That makes забыла especially natural.