Я заметил щель у окна и закрыл её полотенцем.

Заметил is the perfective past tense of заметить, meaning noticed as a completed event.

In this sentence, the speaker is talking about a specific moment:

• first, I noticed the gap,
• then, I closed/covered it.

If you used замечал, that would usually suggest an ongoing, repeated, or habitual idea, like I used to notice or I was noticing, which does not fit as well here.

So:

• заметил = noticed, caught sight of
• замечал = used to notice / was noticing

For the same reason: закрыл is perfective, showing a completed action.

The sentence describes a finished result:

• the speaker noticed the gap,
• then did something to close or cover it.

Закрывал would sound more like:

• was closing,
• used to close,
• kept closing,

which does not match the single completed action here.

So:

• закрыл = closed / covered up completely
• закрывал = was closing / used to close

In the past tense, Russian verbs agree with the gender of the subject.

Here the subject is Я (I), so the verb form depends on whether the speaker is male or female:

• male speaker: Я заметил ... и закрыл ...
• female speaker: Я заметила ... и закрыла ...

So this sentence, as written, implies that the speaker is male.

Щель means gap, crack, or narrow opening.

Here it is the direct object of заметил, so it is in the accusative case.

However, for this noun, the accusative looks the same as the nominative:

• nominative: щель
• accusative: щель

That is normal for many inanimate feminine nouns of this type.

So even though the form does not change, its role in the sentence is still accusative:

• Я заметил щель = I noticed a gap

У окна means by the window, near the window, or at the window.

The preposition у normally requires the genitive case, so:

• окно = window
• у окна = by/near the window

This is a very common pattern:

• у двери = by the door
• у стены = by the wall
• у дома = by the house

So окна is the genitive singular form of окно.

This is a very useful distinction.

• щель у окна = a gap by/near the window
• щель в окне = a gap in the window

So у окна suggests the gap is somewhere around the window area, perhaps where the window frame meets the wall, or where air is getting in around it.

В окне would more strongly suggest that the opening is actually in the window itself.

In this sentence, у окна probably means a drafty gap around the window.

Её means her or it, depending on context. Here it means it, referring back to щель.

Since щель is a feminine noun, the pronoun used to replace it is feminine:

• щель → её

In the sentence:

• закрыл её = closed/covered it

Russian often uses gendered pronouns even for things, unlike English, where we usually just say it.

Also, её here is in the accusative case, because it is the direct object of закрыл.

Yes, it is often written ее in normal printed Russian, but её is the more precise spelling.

The letter ё shows the sound yo. So:

• её is pronounced roughly like yi-YO

In many texts, Russians omit the dots and write ее, expecting the reader to know the correct pronunciation from context.

For learners, writing ё is very helpful.

Полотенцем is the instrumental singular form of полотенце (towel).

The instrumental case is often used to show the means or tool used to do something:

• писать ручкой = to write with a pen
• резать ножом = to cut with a knife
• закрыть полотенцем = to close/cover with a towel

So in this sentence:

• закрыл её полотенцем = covered/closed it with a towel

The towel is the thing used to block the gap, so Russian uses the instrumental.

Because Russian often expresses with simply through the instrumental case, without a separate preposition.

English says:

• with a towel

Russian says:

• полотенцем

This is a very common feature of Russian grammar. The case ending itself gives the meaning of with / by means of.

So:

• закрыл её полотенцем literally works like closed it by means of a towel

In this context, it often means blocked up or covered over, not necessarily shut in the way you shut a door.

Russian закрыть is broader than English close. It can mean:

• shut
• cover
• block
• seal off

So here, because the object is щель and the means is полотенцем, the natural meaning is:

• I blocked/covered the gap with a towel

That is why the sentence makes sense even though a gap is not something you normally shut like a window or door.

Because и is simply joining two verbs with the same subject:

• Я заметил ... и закрыл ...

This is a single sentence with one subject (Я) and two coordinated actions. In Russian, you normally do not put a comma before и in that structure.

You would need a comma if the structure were more complicated, for example with separate clauses that require it.

Yes. Russian word order is relatively flexible, because the cases show the grammatical roles.

The neutral order here is:

• Я заметил щель у окна и закрыл её полотенцем.

But other orders are possible for emphasis, for example:

• У окна я заметил щель и закрыл её полотенцем.
• Щель у окна я заметил и закрыл её полотенцем.

These alternatives may change emphasis:

• У окна first emphasizes location
• Щель first emphasizes what was noticed

The original version sounds natural and straightforward.

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

Whether щель means a gap or the gap is understood from context.

So:

• Я заметил щель can mean I noticed a gap
• in another context it could also be understood as I noticed the gap

English must choose an article, but Russian does not.

Yes. Полотенце is the normal general word for towel.

Its forms are:

• nominative: полотенце
• instrumental: полотенцем

So:

• закрыл её полотенцем = covered it with a towel

This sounds perfectly natural if someone is stuffing or covering a drafty gap with a towel.