Si aer in cubiculo tepidus est, nocte melius dormitur.

This is a simple present conditional:

• Si aer in cubiculo tepidus est = If the air in the bedroom is warm
• nocte melius dormitur = one sleeps better at night

Latin often uses the present indicative in both parts to express something general, habitual, or broadly true. It is not about one single future event only; it sounds more like a general observation.

Aer is in the nominative singular, so it is the subject of est.

Here the first clause is literally structured like this:

• aer = the air
• in cubiculo = in the room / bedroom
• tepidus = warm
• est = is

So the core statement is aer ... tepidus est = the air is warm.

Because tepidus must agree with aer.

Aer is:

• masculine
• singular
• nominative

So the adjective describing it must also be:

• masculine
• singular
• nominative

That gives tepidus.

If the noun were neuter singular nominative, then tepidum would make sense, but aer is masculine.

Because in takes the ablative when it means in or inside a place, with no movement.

So:

• in cubiculo = in the bedroom / in the room

This is different from in + accusative, which usually shows motion into something.

For example:

• in cubiculo est = he/she is in the room
• in cubiculum it = he/she goes into the room

Here there is no movement, so Latin uses the ablative: cubiculo.

It is ablative singular of cubiculum.

You can tell because:

• the preposition in with location takes the ablative
• cubiculum is a second-declension neuter noun
• its ablative singular is cubiculo

So in cubiculo is a very standard Latin phrase for location.

This is the ablative of time when.

Latin often uses the ablative by itself to mean:

• at night
• by day
• in summer
• at that time

So nocte means at night / during the night.

There is no need for a preposition here. This is just a normal Latin idiom.

Melius is the comparative adverb of bene.

• bene = well
• melius = better
• optime = best

Here it modifies dormitur, so it means better in the sense of sleeping better.

It is not an adjective here. It is an adverb.

Because it is passive in form.

Dormitur is:

• present tense
• third person singular
• passive

But with some intransitive verbs like dormio (sleep), Latin can use the impersonal passive. That means the form is passive, but in English we usually translate it actively:

• dormitur = people sleep, one sleeps, there is sleeping

So in this sentence, melius dormitur means one sleeps better or people sleep better, not literally it is slept better in normal English.

Because dormitur is impersonal.

Latin sometimes avoids naming a subject and instead uses a third-person singular passive form to express a general action. English usually has to add something like:

• one
• people
• someone
• sometimes just a natural English verb with no stated subject

So Latin does not need a word for one here. The verb form itself already gives that impersonal sense.

Latin word order is much more flexible than English word order because the endings show how the words function.

English depends heavily on position:

• The air is warm in the room

Latin can move things around more freely:

• Si aer in cubiculo tepidus est, nocte melius dormitur
• Si in cubiculo aer tepidus est, melius nocte dormitur
• and other variations

The original order is natural and clear, but not the only possible order. Often Latin places words where they sound most balanced or where the writer wants emphasis.

In the first clause, est is the main verb: the air is warm.

In the second clause, the whole idea is contained in dormitur, which is already a complete finite verb meaning people sleep / one sleeps in an impersonal way.

So:

• first clause: adjective + verb of being → tepidus est
• second clause: single verb expresses the action → dormitur

Latin often packs a lot into one verb form.

Not necessarily. In this sentence, the present tense expresses a general truth or habitual fact, not just something happening at this exact moment.

So the sense is more like:

• If the air in the bedroom is warm, one sleeps better at night
• When the air in the bedroom is warm, people sleep better at night

That is a very common use of the Latin present tense.