Metto gli occhiali sul comodino prima di dormire.

In Italian, the subject pronoun is often left out because the verb ending already shows who is doing the action.

• metto = I put
• the -o ending tells you it is first person singular

So Metto gli occhiali... already clearly means I put my glasses....
You can say io metto if you want extra emphasis, for example to contrast with someone else.

Metto is the present indicative of mettere.

In a sentence like this, the Italian present tense often describes a habit or routine, not only something happening right now.

So this can mean something like:

• I put my glasses on the nightstand before going to sleep
• I usually put my glasses on the nightstand before sleeping

Here, mettere means to put or to place.

Because the sentence includes a location — sul comodino — the meaning is clearly to place the glasses on the nightstand.

This is useful because mettere gli occhiali by itself can also mean to put on glasses.
But once you add sul comodino, it is clearly about placing them there.

Because occhiali is a masculine plural noun that begins with a vowel.

For masculine plural nouns, Italian uses:

• i before many consonants
• gli before vowels and certain consonant groups

So:

• i libri
• gli amici
• gli occhiali

That is why the correct form is gli occhiali.

In Italian, just like in English, glasses are normally treated as a plural item.

• l’occhiale = one lens / one eyepiece / sometimes a monocle-like item
• gli occhiali = glasses / eyeglasses

So if you mean normal eyeglasses, the usual word is the plural occhiali.

Italian often leaves out the possessive when the owner is obvious from context.

So gli occhiali is often enough to mean my glasses here, because it is obvious that the speaker is talking about their own glasses.

Using i miei occhiali is possible, but it adds emphasis, contrast, or clarity:

• Metto gli occhiali sul comodino = I put my glasses on the nightstand
• Metto i miei occhiali sul comodino, non i tuoi = I put my glasses on the nightstand, not yours

The gli sound is one of the trickier Italian sounds for English speakers.

In gli occhiali, gli is pronounced as a palatal sound, roughly similar to ly in a very smooth, blended way — but not exactly like normal English ly.

A rough approximation is:

• gli occhiali ≈ lyok-KYAH-lee

More accurately, occhiali is stressed on the a:

• oc-CHIA-li

So the whole phrase sounds roughly like:

• lyok-KYAH-lee

Sul means on the.

It is a contraction of:

• su = on
• il = the

So:

• su + il = sul

This is very common in Italian. Similar combinations include:

• nel = in + il
• del = di + il
• al = a + il

Because comodino is masculine singular, sul comodino is the correct form.

Comodino means nightstand or bedside table.

So:

• sul comodino = on the nightstand

Also notice that you do not say sul il comodino.
The article il is already built into sul.

In Italian, prima di + infinitive means before doing something.

So:

• prima di dormire = before sleeping / before going to sleep

This is the normal structure when the subject is the same as in the main clause.

Other examples:

• prima di mangiare = before eating
• prima di uscire = before going out
• prima di studiare = before studying

After prima di, Italian normally uses the infinitive when the person doing both actions is the same.

In this sentence, the same person:

• puts the glasses on the nightstand
• sleeps / goes to sleep

So Italian uses:

• prima di dormire

If the subject changes, Italian usually uses prima che + subjunctive instead:

• Prima che tu dorma, metto via tutto.
  = Before you sleep, I put everything away.

So in your sentence, the infinitive is correct because the subject stays the same.

Yes. Italian word order is fairly flexible, although some versions sound more natural than others depending on what you want to emphasize.

Your sentence is perfectly natural:

• Metto gli occhiali sul comodino prima di dormire.

You could also say:

• Prima di dormire, metto gli occhiali sul comodino.

That version puts more emphasis on the time expression before going to sleep.

So both are correct; the difference is mainly in focus and rhythm, not basic meaning.