Le soir, je mets mon pyjama avant de me coucher.

Here, Le soir means in the evening or in the evenings.

In French, le + a time word often expresses a general habit:

• le matin = in the morning
• l’après-midi = in the afternoon
• le soir = in the evening

So this sentence sounds like a routine, not a one-time event.

Le soir talks about a usual pattern or habit: in the evening / every evening.

Ce soir means this evening / tonight, so it refers to one specific evening.

Compare:

• Le soir, je mets mon pyjama... = this is my routine.
• Ce soir, je mets mon pyjama... = I’m talking about tonight.

Je mets comes from mettre, which means to put on or to put.

So:

• je mets mon pyjama = I put on my pajamas

This focuses on the action of getting dressed for bed.

Because mettre and porter do not mean the same thing.

• mettre = to put on
• porter = to wear

So:

• je mets mon pyjama = I am putting it on
• je porte mon pyjama = I am wearing it

In this sentence, the speaker is describing the action that happens before bed, so mets is the natural choice.

French normally uses pyjama as a singular noun:

• un pyjama
• mon pyjama

Even though English often uses a plural form, French does not copy that pattern. So mon pyjama is completely normal French.

French often uses a possessive adjective for clothing when talking about your own things in a simple, direct way.

So:

• mon pyjama = my pajamas / my pajama set

Using le pyjama would sound less natural here unless the context had already made it very specific.

After avant, French normally uses de before an infinitive:

• avant de partir = before leaving
• avant de dormir = before sleeping
• avant de me coucher = before going to bed

So avant de + infinitive is the standard pattern.

The full verb is se coucher, which means to go to bed or to lie down.

It is reflexive because the action is understood as happening to the subject themself:

• je me couche = I go to bed
• tu te couches = you go to bed

French uses many reflexive verbs in places where English does not, so this is a very common pattern to get used to.

In French, reflexive pronouns go directly before the verb they belong to, even when that verb is an infinitive.

So:

• je me couche = I go to bed
• avant de me coucher = before going to bed

French does not put it after the verb the way English might suggest.

Because avant de + infinitive is the normal structure when the subject is the same in both parts of the sentence.

Here, the same person does both actions:

• je mets mon pyjama
• je me couche

So French prefers:

• avant de me coucher

You would use avant que when there is a separate clause, often with a different subject:

• Je mets mon pyjama avant que les invités arrivent.

Also, avant que is followed by the subjunctive.

Je mets is the present tense of mettre.

French, like English, uses the present tense to describe habits and routines:

• Le soir, je mets mon pyjama... = this is what I usually do in the evening.

So the sentence is not necessarily happening right now; it is describing a regular action.

Yes. That is also correct.

Both versions are natural:

• Le soir, je mets mon pyjama...
• Je mets mon pyjama le soir...

Putting Le soir first gives a little more emphasis to the time expression and sounds very natural when describing a routine.

Yes. A common alternative is:

• avant d’aller au lit = before going to bed

So you could also say:

• Le soir, je mets mon pyjama avant d’aller au lit.

Both are correct and natural.
Se coucher is a bit more directly about the act of going to bed, while aller au lit is literally to go to bed.

Yes, a few:

• mets is pronounced like may; the t is silent.
• je sounds like zhuh
• pyjama is usually pronounced pee-zha-ma
• coucher ends with an -ay sound: koo-shay

A rough guide to the whole sentence is:

Le soir, je mets mon pyjama avant de me coucher
≈ luh swar, zhuh may mon pee-zha-ma ah-vahn duh muh koo-shay