Aerul este rece.

Because aerul means the air, while aer means just air.

Romanian usually puts the definite article at the end of the noun instead of using a separate word like English the. So:

• aer = air
• aerul = the air

The ending -ul is the definite article here.

Este means is.

It is the 3rd person singular present form of a fi = to be.

So the structure is:

• aerul = the air
• este = is
• rece = cold

Yes. That is very common.

E is the short everyday form of este, so:

• Aerul este rece = correct
• Aerul e rece = also correct, and very natural in speech

For a learner, it is good to recognize both forms.

Because rece is a predicate adjective here: it comes after the verb to be and describes the subject.

This is very similar to English:

• The air is cold
• Aerul este rece

So the basic pattern is:

subject + to be + adjective

Because that would not be the normal way to build this sentence.

If you want a full sentence meaning The air is cold, Romanian normally says:

• Aerul este rece

If you want cold air as a noun phrase, Romanian usually puts the adjective after the noun:

• aer rece = cold air

So compare:

• aer rece = a noun phrase
• aerul este rece = a full sentence

Yes, but not in this sentence.

Rece is the singular form, and it works for both masculine/neuter singular and feminine singular. In the plural, it becomes reci.

For example:

• aerul este rece = the air is cold
• apa este rece = the water is cold
• zilele sunt reci = the days are cold

So in your sentence, rece is the correct singular form.

Aer is a neuter noun in Romanian.

That can be confusing at first, because Romanian neuter nouns act like masculine in the singular and feminine in the plural. In this sentence, the singular definite form is aerul, which looks like a masculine form.

So even though aer is neuter, aerul is exactly what you should expect in the singular.

Normally, no.

In standard Romanian, you need the verb to be in a sentence like this:

• Aerul este rece
• Aerul e rece

If you say only aerul rece, that usually means the cold air, not a complete sentence.

So este or e is needed if you want to say The air is cold.

Yes, but Aerul este rece is the normal, neutral order.

You might sometimes hear something like:

• E rece aerul

But that is more marked and depends on context, usually for emphasis. As a learner, the safest pattern is:

Aerul e rece / Aerul este rece

A rough English-friendly guide is:

a-e-rul es-te re-che

A few useful pronunciation notes:

• aerul has three syllables: a-e-rul
• c before e sounds like ch, so rece sounds roughly like re-che
• Romanian r is usually tapped or lightly rolled
• este is often pronounced es-te, though in natural speech some speakers say something closer to yes-te

You do not need perfect pronunciation right away; the most important thing is noticing that rece does not have a hard k sound.

Because rece is the normal adjective for describing something as cold.

So:

• Aerul este rece = The air is cold
• Apa este rece = The water is cold

Frig is usually used differently, especially in expressions like:

• E frig = It is cold

That expression talks more about the weather or general temperature, not about describing a noun directly in the same way. So for the air is cold, rece is the natural choice.