Когда я заканчиваю подметать пол и убираю иголку с ниткой, мне хочется просто сесть у окна и отдохнуть.

In Russian, the present tense is often used for repeated, habitual situations.

So this sentence means something like:

• Whenever / When I finish sweeping the floor and put away the needle and thread, I feel like...

The speaker is not talking about one specific future occasion, but about a typical situation.

If it were about one specific future time, Russian would more likely use perfective future forms:

• Когда я закончу подметать пол и уберу иголку с ниткой...

So here the present tense helps show a regular pattern.

After заканчивать / закончить, Russian usually uses an infinitive to say what activity is being finished.

So:

• заканчиваю подметать = I am finishing sweeping
• literally, I finish to sweep is not good English, but that is the idea structurally

You cannot normally say:

• заканчиваю подметаю ❌

because подметаю is a finite verb form (I sweep / am sweeping), while заканчиваю needs an activity name, and the infinitive fills that role.

After заканчивать / закончить, Russian usually prefers the imperfective infinitive:

• заканчивать подметать
• закончить читать
• закончить писать

That is because the infinitive names the process/activity that is being brought to an end.

So подметать is natural here because it means to be sweeping / to sweep as an activity.

Using подмести here would sound much less natural, because подмести is perfective and focuses on the completed result, not the ongoing activity.

Пол is the direct object of подметать, so it is in the accusative case.

But for many inanimate masculine nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: пол
• accusative: пол

That is why you do not see a change here.

There are two different case patterns here:

• иголку is the direct object of убираю, so it is in the accusative
• с ниткой means with thread, so after с in this meaning, Russian uses the instrumental

So:

• иголка → иголку
• нитка → ниткой

This phrase literally means a needle with thread.

Both are possible words, but they are not quite the same in feel.

• нитка = a thread, a piece of thread, everyday word
• нить = thread, but often a bit more formal/bookish or abstract depending on context

In a practical household context like sewing, иголка с ниткой is the most natural everyday phrase.

Also, the instrumental forms are:

• нитка → ниткой / ниткою
• нить → нитью

So с ниткой sounds very normal here.

Here убираю means something like put away, tidy away, or clear away.

The verb убирать / убрать is very broad and can mean:

• to clean up
• to remove
• to put away
• to tidy up

In this sentence, with иголку с ниткой, the sense is most naturally:

• I put away the needle and thread

Мне хочется is an impersonal way to say I feel like... / I want to...

Literally it is closer to:

• To me, it is wanted
• or more naturally: I have the urge / I feel like

Compared with я хочу, it often sounds:

• softer
• more spontaneous
• more like a feeling in the moment

So:

• я хочу сесть у окна = I want to sit by the window
• мне хочется сесть у окна = I feel like sitting by the window

Both are correct, but мне хочется fits the mood of this sentence very well.

Because хочется is followed by an infinitive to say what the person feels like doing.

The pattern is:

• кому?
  • хочется
    • infinitive

So here:

• мне = to me
• хочется = I feel like
• сесть, отдохнуть = the actions desired

This is a very common Russian structure:

• Мне хочется спать.
• Ему хочется поесть.
• Нам хочется уйти.

The perfective forms fit because the speaker is thinking of specific whole actions:

• сесть = to sit down
• отдохнуть = to have a rest / rest a bit

The idea is not the ongoing process, but the natural sequence:

1. sit down
2. rest

If you used the imperfective forms, the focus would shift more toward process or habitual action:

• садиться
• отдыхать

Those are possible in some contexts, but here the perfective forms sound more natural because they express the speaker’s immediate wish to do those complete actions after finishing the chores.

У окна means by the window or near the window.

The preposition у takes the genitive case, so:

• окно → окна

So:

• у окна = by the window

This is a very common expression for location near something:

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

Because the sentence begins with a subordinate clause introduced by Когда:

• Когда я заканчиваю... и убираю..., мне хочется...

Russian separates that subordinate clause from the main clause with a comma.

So the structure is:

• Когда ... , main clause

This is very similar to English:

• When I finish..., I feel like...

Yes. Russian word order is flexible.

This sentence could be rearranged, for example:

• Мне хочется просто сесть у окна и отдохнуть, когда я заканчиваю подметать пол и убираю иголку с ниткой.

That is grammatically possible, but the original order is more natural because it presents the sequence clearly:

1. first the chores are finished
2. then the feeling arises

The original version also sounds smoother and more natural in storytelling or description.