Bitte puste auf die Suppe, sie ist noch zu heiß.

Because this is a command. German uses the imperative for commands, requests, and instructions.

The full statement would be du pustest auf die Suppe = you blow on the soup, but the command form drops du and changes the verb:

• du pustest → puste!

So Bitte puste auf die Suppe means Please blow on the soup.

This is the informal singular command, used when speaking to one person as du.

That -e is a normal imperative ending in German, especially when it makes the form easier to say.

With pusten, the stem is pust-, and puste! sounds smoother than just pust!. You may hear both, but puste! is very natural here.

So:

• puste! = standard, clear, natural
• pust! = shorter, more abrupt/colloquial

Not by itself. Bitte just means please and makes the command more polite.

The sentence is still informal, because puste is the du imperative.

Compare:

• Bitte puste auf die Suppe. = please blow on the soup. (informal, one person)
• Bitte pusten Sie auf die Suppe. = please blow on the soup. (formal)
• Bitte pustet auf die Suppe. = please blow on the soup. (informal plural)

Because German normally says auf etwas pusten or auf etwas blasen for to blow on something.

So the preposition auf is part of the usual expression here:

• auf die Suppe pusten = to blow on the soup

This matches English fairly closely, since English also says blow on the soup.

Here auf takes the accusative.

German prepositions like auf can sometimes take different cases, but with pusten auf the action is directed onto/toward something, so accusative is used.

Since Suppe is a feminine noun:

• nominative: die Suppe
• accusative: die Suppe
• dative: der Suppe

That is why you see auf die Suppe.

Here sie means it, referring to die Suppe.

In German, pronouns follow the grammatical gender of the noun:

• die Suppe is feminine
• so the pronoun is sie

Even though sie can also mean she or they in other contexts, here it clearly means the soup / it.

Here noch means still.

So:

• sie ist noch zu heiß = it is still too hot

The idea is that the soup has not cooled down enough yet.
Without noch, the sentence would simply say it is too hot. With noch, it emphasizes that this is still the case at the moment.

Zu heiß means too hot — hotter than is acceptable or comfortable.

Sehr heiß means very hot — strong description, but not necessarily a problem.

So:

• Die Suppe ist sehr heiß. = The soup is very hot.
• Die Suppe ist zu heiß. = The soup is too hot.

In this sentence, zu heiß is used because the heat is a problem: the person should blow on the soup before eating it.

Because it is a normal main clause, and German main clauses usually follow the verb-second rule.

The parts are:

• sie = subject
• ist = verb
• noch zu heiß = rest of the predicate

So the verb ist comes in the second position:

• sie ist noch zu heiß

That is standard German main-clause word order.

Because the sentence contains two closely connected main clauses:

• Bitte puste auf die Suppe
• sie ist noch zu heiß

The second clause explains the reason for the first one. German often links such clauses with a comma.

You could also say:

• Bitte puste auf die Suppe, denn sie ist noch zu heiß.
• Bitte puste auf die Suppe. Sie ist noch zu heiß.

All of these are natural, with slightly different rhythm and style.

Yes. Blasen also means to blow.

For example:

• Bitte blas auf die Suppe.

That is understandable and natural too.

The difference is mostly one of usage and tone:

• pusten often sounds a bit more everyday, gentle, or child-directed
• blasen is more general and broader in meaning

In a sentence about cooling soup, both work, but pusten feels especially fitting.