Nokte mi aŭdis la pluvon frapi la fenestron, sed matene la ĉielo denove brilis.

They are adverbs. In Esperanto, -e makes an adverb, so nokte means at night / during the night, and matene means in the morning.

They come from the noun roots nokt- and maten-. This is a very common way to express time:

• tage = by day, during the day
• vespere = in the evening
• somere = in summer

So Nokte mi aŭdis... is literally something like At night I heard...

Yes. All of these can work, but they are not identical in feel.

• nokte = at night, night-time, a simple adverbial time expression
• en la nokto = in the night
• dum la nokto = during the night

In many everyday sentences, nokte is the most compact and natural choice. The same applies to matene.

Because la pluvon is the direct object of aŭdis.

The sentence structure is:

• mi aŭdis = I heard
• la pluvon = the rain

So the -n shows what was heard.

Even though the rain is also understood as the thing doing the action frapi, it still takes -n because grammatically it is the object of the main verb aŭdis.

Because after verbs of perception such as aŭdi and vidi, Esperanto often uses an object + infinitive construction.

So:

• Mi aŭdis la pluvon frapi la fenestron.
• literally: I heard the rain hit the window.

This is very similar to English patterns like:

• I heard the baby cry.
• I saw him leave.

If you said frapis, that would create a separate finite clause and would need a different structure.

In meaning, it is the doer of frapi. But in grammar, it is the object of aŭdis.

That is exactly why this construction can feel a little unusual to English speakers at first.

Think of it this way:

• main clause: mi aŭdis la pluvon
• added action perceived: frapi la fenestron

So la pluvon is the thing heard, and also the thing understood to be hitting the window.

Because fenestron is the direct object of frapi.

So inside the infinitive phrase:

• frapi = to hit
• la fenestron = the window

That gives:

• la pluvon frapi la fenestron = the rain hitting the window

So there are two accusatives in the sentence, but they belong to different verbs:

• pluvon goes with aŭdis
• fenestron goes with frapi

Because all three are understood as definite.

• la pluvon = the rain that was falling, the rain I could hear
• la fenestron = the window in question
• la ĉielo = the sky, which is naturally a unique and definite thing in the scene

Esperanto uses la much like English the. In this sentence, all three nouns are specific enough to sound natural with the article.

Because la ĉielo is the subject of brilis.

• la ĉielo = the sky
• brilis = shone / was shining / gleamed

The subject does not take -n. Only direct objects do.

Denove means again or anew.

So la ĉielo denove brilis means that the sky was shining again after the rainy night.

It adds the idea of a return to a previous state: the bad weather passed, and brightness came back.

Esperanto uses -is for past tense in general. It does not force the same kind of distinction English often makes between simple past and past progressive.

So:

• aŭdis can mean heard or sometimes was hearing, depending on context
• brilis can mean shone or was shining

Usually the context makes the meaning clear. If you need extra precision, you can add words or rephrase, but very often plain -is is enough.

The given word order is natural, but Esperanto word order is fairly flexible.

This sentence begins with Nokte and later matene to highlight the time contrast:

• at night ...
• in the morning ...

That makes the sentence flow nicely.

You could change the order somewhat, for example:

• Mi nokte aŭdis la pluvon frapi la fenestron...

But the original version sounds very good because it emphasizes the contrast between the two times of day.

Frapi la fenestron is perfectly understandable and natural here. It gives the image of the rain striking or tapping the window.

You could also find expressions like:

• bati kontraŭ la fenestro
• frapi kontraŭ la fenestro

Those emphasize contact against the window.

The original sentence is fine and idiomatic. It sounds vivid and slightly poetic, which fits the image well.

Two things often catch English speakers here:

• aŭ in aŭdis is a diphthong, roughly like the ow in cow
• ĉ in ĉielo sounds like ch in church

So approximately:

• aŭdis ≈ OW-dis
• ĉielo ≈ chee-EH-lo, with the stress on EH

Remember that Esperanto stress normally falls on the second-to-last syllable:

• aŭ-DIS
• ĉi-E-lo

Yes, but that would be a different structure.

For example:

• Mi aŭdis, ke la pluvo frapis la fenestron.

That means something more like I heard that the rain hit the window, which can suggest receiving information, not necessarily directly perceiving the sound.

But:

• Mi aŭdis la pluvon frapi la fenestron.

means you directly heard the rain hitting the window.

So the original sentence is especially good if the idea is literal hearing, not just learning a fact.