Avaa ikkuna ja päästä kuuma ilma ulos.

Avata is the dictionary form, meaning to open.

Avaa is the second-person singular imperative, meaning open!

So:

• avata = to open
• avaa = open!

In this sentence, the speaker is giving a command, so the imperative form is needed.

This is a very common Finnish object-case question.

In an affirmative singular imperative, a total object is usually in the nominative form, not the genitive-looking form.

So:

• Avaan ikkunan. = I open the window.
• Avaa ikkuna. = Open the window!

That is why you see ikkuna, not ikkunan.

Finnish does not have articles like English a/an and the.

So ikkuna can mean:

• a window
• the window

And kuuma ilma can mean:

• hot air
• the hot air

The exact meaning comes from context, not from a separate article word.

Yes, the dictionary form is päästää, meaning to let out, release, let go.

But in a command, the form changes:

• päästää = to let out
• päästä = let out!

So in this sentence, päästä is the imperative of päästää.

This can be confusing because there is also a different verb päästä, which means things like to get to, to escape, or to be allowed to go. Here, though, the meaning is clearly from päästää, because it is a command and it has an object: kuuma ilma.

For the same reason as ikkuna above: this is a total object in an affirmative imperative, so it appears in the nominative.

So:

• Päästä kuuma ilma ulos. = Let the hot air out.

If you used kuumaa ilmaa, that would be the partitive, which often suggests an incomplete, ongoing, or indefinite amount of something.

Also notice that the adjective agrees with the noun:

• kuuma ilma = nominative
• kuumaa ilmaa = partitive
• kuuman ilman = genitive-like form

Ulos means out or to the outside.

It shows direction: something moves from inside to outside.

So päästää ulos is a very common combination meaning to let out.

Without ulos, päästää would be more general, like release or let go.
With ulos, the meaning is specifically let out.

Finnish usually does not include the subject pronoun when the verb form already shows who the subject is.

In commands, this is especially normal.

So:

• avaa already means open!
• päästä already means let out!

The you is understood automatically.
Adding sinä would usually sound unnecessary unless you wanted special emphasis.

The word ilma can mean both air and weather.

So on its own, kuuma ilma could in some contexts mean hot weather. But in this sentence, the context makes the meaning clear:

• Avaa ikkuna = open the window
• päästä ... ulos = let ... out

That clearly points to hot air, not hot weather.

This is a normal and natural order.

Finnish word order is more flexible than English, but that does not mean every order sounds equally natural. Here, Avaa ikkuna ja päästä kuuma ilma ulos is a good neutral model.

Especially in päästää ... ulos, it is very common to place the object before ulos:

• päästä kuuma ilma ulos

A different order may be possible in special contexts, but this one is the safest and most standard for a learner.

Then you would use the plural imperative:

• Avatkaa ikkuna ja päästäkää kuuma ilma ulos.

So:

• avaa / päästä = command to one person
• avatkaa / päästäkää = command to more than one person

English does not change the command form for singular and plural, but Finnish does.