Ventileer die kamer mits die lug buite koeler is.

Yes. Ventileer is the verb, and here it is in the imperative form, so the sentence is giving a command or instruction.

In Afrikaans, commands often begin with the bare verb:

• Ventileer die kamer. = Ventilate the room.
• Wag hier. = Wait here.
• Luister mooi. = Listen carefully.

There is no separate word for you in this kind of command.

Because this is an imperative sentence. In Afrikaans, just like in English, the subject you is usually omitted in commands.

So:

• Ventileer die kamer literally functions like [You] ventilate the room.

If you want to make it softer or more polite, you can add something like:

• Ventileer asseblief die kamer. = Please ventilate the room.

Mits means provided that, as long as, or sometimes simply if in a conditional sense.

It introduces a condition that must be true for the action in the main clause to apply.

So in this sentence, the idea is:

• ventilate the room provided that the outside air is cooler.

A learner should know that mits is often a bit more formal or written than a plain everyday if.

Because mits introduces a subordinate clause, and in Afrikaans, the finite verb usually goes to the end of a subordinate clause.

Compare:

• Main clause: Die lug buite is koeler.
• Subordinate clause: ... mits die lug buite koeler is.

That final is is very typical Afrikaans word order after words like:

• as
• omdat
• wanneer
• indien
• mits

Koeler is the normal comparative form of koel.

Just as English has:

• cool → cooler

Afrikaans has:

• koel → koeler

So koeler means cooler.

Afrikaans often forms comparatives with -er:

• groot → groter
• klein → kleiner
• warm → warmer
• koel → koeler

Using meer koel would usually sound unnatural here.

Literally, die lug buite means the air outside.

Here, buite is an adverb meaning outside, but it is being used after die lug to specify which air we mean: the air that is outside.

This is a natural Afrikaans pattern:

• die mense daar = the people there
• die huis oorkant = the house opposite/across the way
• die lug buite = the air outside

So buite is not strange here; it is simply narrowing down die lug.

Yes. Die here is the definite article, so it means the.

Afrikaans uses:

• die = the
• ’n = a/an

Unlike English, Afrikaans does not change the word for gender, and die is used very widely.

So:

• die kamer = the room
• die lug = the air

It is natural, but it sounds a bit formal, instructional, or written, especially because of mits and ventileer.

In more everyday speech, people might say something like:

• Lug die kamer as die lug buite koeler is.
• Maak die vensters oop as dit buite koeler is.

So the given sentence is good Afrikaans, but it sounds more like advice in a guide, notice, or instruction.

In formal Afrikaans punctuation, you will often see a comma before a subordinate clause like this:

• Ventileer die kamer, mits die lug buite koeler is.

So many learners would expect a comma there. If you see the sentence without one, that may be due to a simplified example, informal writing, or inconsistent punctuation. For learning purposes, it is useful to recognize that a comma is commonly used before this kind of clause.

Yes. You can put the conditional clause first:

• Mits die lug buite koeler is, ventileer die kamer.

That is grammatically fine. When the subordinate clause comes first, the main clause follows it.

This fronted version may sound even more formal, but it is correct.

No, not here. Afrikaans still needs the verb is in this clause because koeler is a predicate adjective, and the clause needs the copula:

• Die lug buite is koeler.

So in the subordinate clause, that becomes:

• ... mits die lug buite koeler is.

Leaving out is would make the sentence ungrammatical.

Not exactly. Ventileer means ventilate or air out, so it refers to improving airflow in the room. Opening a window is one way to do that, but the word itself does not specifically mean open the window.

So:

• Ventileer die kamer = make sure the room gets fresh air / ventilate the room
• Maak die venster oop = open the window

They are related ideas, but not the same expression.