Я пользуюсь зубной нитью каждый вечер, чтобы между зубами не оставалась еда.

Пользуюсь is the 1st person singular form of the verb пользоваться, which means to use in the sense of make use of something.

So:

• я пользуюсь = I use
• infinitive: пользоваться

A very important point is that пользоваться is used with the instrumental case, not the accusative.

So Russian says:

• пользоваться зубной нитью = to use dental floss

not:

• пользоваться зубную нить

This verb is very common for things like:

• пользоваться телефоном = to use a phone
• пользоваться словарём = to use a dictionary
• пользоваться компьютером = to use a computer

Because the verb пользоваться requires the instrumental case.

The dictionary form is:

• зубная нить = dental floss

But after пользоваться, it changes to instrumental:

• зубной нитью

Breakdown:

• зубная → зубной
• нить → нитью

So the sentence literally follows the pattern:

• I use what?
  not in Russian here

but rather:

• I make use of what? → зубной нитью

That is why the noun phrase is in the instrumental.

Yes. Зубная нить is the standard way to say dental floss.

Literally:

• зубная = dental / tooth-related
• нить = thread

So it literally means something like dental thread.

You may also hear expressions based on the verb чистить:

• чистить зубы нитью = to clean your teeth with floss

But пользоваться зубной нитью is very natural and common.

Каждый вечер means every evening.

Here каждый вечер is in the accusative case, which is commonly used in Russian for expressions of repeated time without a preposition.

Compare:

• каждый день = every day
• каждую неделю = every week
• каждый вечер = every evening

Why does вечер look unchanged? Because for this masculine inanimate noun, the accusative singular looks the same as the nominative singular:

• nominative: вечер
• accusative: вечер

But with a feminine noun you can see the change more clearly:

• каждая неделя → каждую неделю

So каждый вечер is a standard time expression meaning every evening.

Because чтобы introduces a subordinate clause.

The first part is:

• Я пользуюсь зубной нитью каждый вечер
  = I use dental floss every evening

The second part explains the purpose:

• чтобы между зубами не оставалась еда
  = so that food does not remain between the teeth

In Russian, subordinate clauses introduced by words like что, когда, если, чтобы are normally separated by a comma.

So the comma here is required.

This is a very common thing that confuses learners.

After чтобы, Russian often uses a form that looks like the past tense, but the meaning is not past. It expresses purpose, desired result, or something like the English idea so that / in order that.

So:

• чтобы ... не оставалась еда
  means
• so that food would not remain / so that food does not remain

The form оставалась is used because Russian forms this construction with чтобы + past-tense-form.

Important points:

• it is not describing the past here
• it is part of the purpose clause
• English usually translates it with so that, so ... doesn’t, or so ... won’t

So although оставалась looks past, the whole clause is really about the intended result of flossing.

Because the verb must agree with еда.

The noun еда is:

• feminine
• singular

In the past-tense-looking form used after чтобы, the verb agrees in gender and number:

• оставался = masculine singular
• оставалась = feminine singular
• оставалось = neuter singular
• оставались = plural

Since еда is feminine singular, we get:

• еда не оставалась

So the ending -ась here reflects both the verb form and the feminine subject.

Because after чтобы, Russian normally uses this special past-form-based construction, not the ordinary present tense.

Compare:

• Между зубами не остаётся еда.
  = Food does not remain between the teeth.
  This is a plain statement.

But in your sentence, the speaker is saying why they floss:

• ...чтобы между зубами не оставалась еда
  = ...so that food does not remain between the teeth

So this is not just a fact; it is a purpose/result clause. That is why не оставалась is used.

Here между means between, and in this sentence it is used for location.

With location, между takes the instrumental case:

• между зубами = between the teeth

The dictionary form is:

• зуб = tooth
• plural nominative: зубы
• plural instrumental: зубами

So:

• между + instrumental

Examples:

• между домами = between the houses
• между друзьями = among/between friends
• между зубами = between the teeth

Because Russian naturally uses the plural here: between the teeth = между зубами.

That is simply the normal way to express the space among the teeth in general. Russian does not need a possessive like my here, because the context already makes it obvious whose teeth are meant.

So:

• между зубами sounds natural
• между моими зубами is grammatically possible, but usually unnecessary and less natural here

Russian often leaves out possessives when the meaning is clear from context.

Because оставаться means to remain / to be left.

That is exactly the idea needed here:

• еда не оставалась = food would not remain / be left

Russian does not use a verb like to be in the same way English sometimes does. So you would not say something equivalent to food is not between the teeth if your real meaning is food doesn’t stay/get left there.

The verb оставаться is a very natural choice for this kind of sentence.

Yes, that would also be natural:

• чтобы еда не застревала между зубами
  = so that food doesn’t get stuck between the teeth

The difference is subtle:

• не оставалась = does not remain / is not left behind
• не застревала = does not get stuck

So:

• оставаться focuses on remaining
• застревать focuses on getting stuck

Your original sentence is perfectly natural and slightly broader in meaning.

Yes. Russian often omits subject pronouns when they are clear from the verb form.

So both are possible:

• Я пользуюсь зубной нитью каждый вечер...
• Пользуюсь зубной нитью каждый вечер...

The first version with Я is still completely normal. It may sound a little more explicit or contrastive depending on context.

For example:

• Я пользуюсь зубной нитью каждый вечер, а брат нет.
  = I use dental floss every evening, but my brother doesn’t.

So the pronoun is not required, but it is not wrong at all.

No, Russian word order is fairly flexible, though some orders sound more neutral than others.

Your sentence is very natural and neutral:

• Я пользуюсь зубной нитью каждый вечер, чтобы между зубами не оставалась еда.

You could also hear variations like:

• Каждый вечер я пользуюсь зубной нитью, чтобы между зубами не оставалась еда.

That puts more emphasis on every evening.

Or:

• Я каждый вечер пользуюсь зубной нитью...

Also natural.

The original order is a good standard version for learners.

Yes. Пользоваться is imperfective, and оставаться is also imperfective.

That matters because the sentence describes:

1. a habit

   • Я пользуюсь ... каждый вечер = I use ... every evening

2. an ongoing/general result

   • чтобы ... не оставалась еда = so that food does not remain

Russian normally uses the imperfective for repeated actions, habits, and general processes.

If you tried to force perfective verbs here, the sentence would sound much less natural because the meaning is not about one single completed event, but about a regular routine and its usual purpose.