Закрой крышку кастрюли, чтобы суп не остывал слишком быстро.

Закрой is the perfective imperative of закрыть and usually means close it now / do it as a single completed action.

In this sentence, the speaker wants one concrete result: put the lid on the pot. That is why perfective sounds natural.

Compare:

• Закрой крышку кастрюли = Close the pot lid / Put the lid on the pot
• Закрывай крышку кастрюли = more like Close the lid (go ahead, start doing it / as a repeated or ongoing instruction)

For a one-time command with a clear result, Russian often prefers the perfective imperative.

Because крышка is the direct object of закрой.

The verb закрыть takes an object in the accusative:

• закрыть дверь
• закрыть окно
• закрыть крышку

Since крышка is a feminine noun, its accusative singular is крышку.

So:

• nominative: крышка
• accusative: крышку

Крышку кастрюли literally means the lid of the pot.

Here:

• крышку = the lid
• кастрюли = of the pot

Russian often uses the genitive to show possession or association, where English often uses of or a noun-noun combination.

So:

• крышка кастрюли = the pot lid / the lid of the pot
• ручка двери = the door handle
• страница книги = the page of the book

Кастрюли is genitive singular of кастрюля.

Here чтобы means so that or in order that.

It introduces the purpose of the action:

• Закрой крышку кастрюли = close the lid
• чтобы суп не остывал слишком быстро = so that the soup does not cool down too quickly

So the whole sentence gives:

1. a command
2. the reason or purpose for it

This is a very common Russian pattern:

• Скажи мне, чтобы я не забыл.
• Напиши это, чтобы все поняли.

Because остывал is imperfective past, and after чтобы it often expresses a desired ongoing situation or prevention of a process.

The idea is not just:

• so that the soup doesn’t become cold at one moment

but rather:

• so that the soup doesn’t cool down too quickly as a process

That is why не остывал слишком быстро sounds natural: it focuses on the cooling as something gradual.

Compare the nuance:

• чтобы суп не остыл = so that the soup wouldn’t get cold / end up cold
• чтобы суп не остывал слишком быстро = so that the soup wouldn’t be cooling down too quickly

Both are possible in some contexts, but they emphasize different things.

After чтобы, Russian very often uses forms that look like the past tense, even when the meaning is about a desired or hypothetical future situation.

So чтобы суп не остывал does not mean the soup was cooling in the past. It means something like:

• so that the soup would not cool
• so that the soup wouldn’t be cooling

This is a normal Russian structure. You will often see:

• чтобы он пришёл
• чтобы она не опоздала
• чтобы дети спали

These are past-form verbs grammatically, but semantically they often refer to a desired, intended, or hypothetical result.

Yes. Слишком быстро means too quickly / too fast.

• быстро = quickly
• слишком = too, excessively

Together they modify остывал:

• не остывал слишком быстро = didn’t cool down too quickly

You can build many similar phrases:

• слишком медленно = too slowly
• слишком громко = too loudly
• слишком рано = too early

In informal speech, yes, чтобы is often shortened to чтоб.

So you may hear:

• Закрой крышку кастрюли, чтоб суп не остывал слишком быстро.

But in neutral written Russian, чтобы is the standard full form and is usually better for learners to use.

Because чтобы introduces a subordinate clause.

The sentence has two parts:

• Закрой крышку кастрюли
• чтобы суп не остывал слишком быстро

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

So the comma here is required.

Sometimes yes, if the context makes it obvious.

For example, in a kitchen situation, someone might say:

• Закрой крышку кастрюли, чтобы не остывал слишком быстро.

This can sound a bit less explicit, because the verb form остывал is masculine singular and implies some understood thing like суп or бульон.

But the full version with суп is clearer and more natural for learners.

Остывал comes from the imperfective verb остывать.

Its perfective partner is usually остыть.

So:

• imperfective: остывать = to be cooling down / to cool down as a process
• perfective: остыть = to cool down completely / to become cool

Examples:

• Суп остывает. = The soup is cooling down.
• Суп остыл. = The soup cooled down / has gone cold.

In your sentence, the imperfective emphasizes the process and speed of cooling.

Yes, somewhat. Russian word order is more flexible than English, though not completely free.

The neutral order here is:

• Закрой крышку кастрюли, чтобы суп не остывал слишком быстро.

You could also hear variations like:

• Чтобы суп не остывал слишком быстро, закрой крышку кастрюли.

That version puts the purpose first and may sound slightly more explanatory or emphatic.

But the original order is very natural for a direct instruction followed by the reason.

In real usage, it can mean either depending on context, and English may translate it differently.

With a pot lid, закрыть крышку кастрюли usually means:

• cover the pot with the lid
• put the lid on / close the lid

Russian focuses on the result that the pot becomes closed/covered.

So even if English prefers put the lid on the pot, Russian can naturally say закрыть крышку кастрюли.