Chaque fois que je vois mon copain au portail, je sais que nous rentrerons ensemble.

Yes. Chaque fois que means every time or each time that.

• chaque = each/every
• fois = time/occasion
• que = that/when in this kind of structure

So Chaque fois que je vois mon copain... means Every time I see my boyfriend/friend...

It introduces a repeated situation, not a one-time event.

French usually uses the present tense after expressions like quand, lorsque, and chaque fois que, even when English might use the present to talk about the future or a repeated future situation.

So:

• Chaque fois que je vois mon copain... = Every time I see my friend/boyfriend...

French does not normally say Chaque fois que je verrai here.

This is very natural in French: the clause after chaque fois que is treated as a general repeated condition.

Vois is the 1st person singular present form of voir (to see).

Conjugation in the present:

• je vois = I see
• tu vois = you see
• il/elle/on voit = he/she/one sees
• nous voyons = we see
• vous voyez = you see
• ils/elles voient = they see

So je vois mon copain simply means I see my friend/boyfriend.

It can mean either, depending on context.

• copain can mean friend, especially for a male friend
• it can also mean boyfriend

So mon copain may be:

• my friend
• my boyfriend

Context tells you which meaning is intended. In everyday French, copain/copine is often a bit more informal than ami/amie.

Because à + le contracts to au in French.

So:

• à + le = au
• à + les = aux

That means:

• au portail = at the gate
• not à le portail

This contraction is required in standard French.

Portail usually means a gate, often a large outside gate, such as:

• a school gate
• a building gate
• a property entrance gate

It is not usually the same as a normal indoor door.
So au portail suggests meeting someone at the gate or entrance.

Rentrerons is the future simple form of rentrer for nous:

• nous rentrerons = we will go back / we will return / we will go home

The speaker is talking about what will happen after seeing the friend:

• je sais que nous rentrerons ensemble = I know that we will go back/home together

So the sentence combines:

• a repeated present situation: Chaque fois que je vois...
• a future result: nous rentrerons ensemble

Because je sais que introduces something the speaker knows will happen next, so French uses the future here.

Compare:

• nous rentrons ensemble = we go/are going home together
• nous rentrerons ensemble = we will go home together

In this sentence, the idea is: when I see him there, I know what is going to happen afterward. So the future tense fits naturally.

Not exactly.

Rentrer often means:

• to go back
• to return
• to go home
• to come back inside

It often suggests returning to a usual place, especially home.

Retourner also means to return/go back, but it is often more neutral and less tied to the idea of going home or re-entering.

So in this sentence, rentrerons ensemble suggests something like:

• we’ll go home together
• we’ll head back together

Both are possible in French, but they have different tones.

• nous rentrerons = more standard, slightly more formal or careful
• on rentrera = very common in everyday spoken French

So the sentence could also be said informally as:

• Chaque fois que je vois mon copain au portail, je sais qu’on rentrera ensemble.

The original version with nous is completely natural, especially in written or more neutral French.

Ensemble means together.

It tells you how they will return:

• nous rentrerons = we will go back
• nous rentrerons ensemble = we will go back together

Putting it at the end is very natural in French.

Because French normally keeps a straightforward order in this kind of clause:

• je sais = I know
• que = that
• nous rentrerons = we will return/go home
• ensemble = together

So literally:

• I know that we will go back together

French usually does not move ensemble earlier unless there is a special emphasis.

Yes. The comma separates the introductory clause from the main clause:

• Chaque fois que je vois mon copain au portail,
• je sais que nous rentrerons ensemble.

This makes the structure clearer:

1. repeated situation: Every time I see my friend at the gate
2. result/knowledge: I know that we will go back together

The comma is very natural here, just as in English.

Yes. A few natural alternatives are:

• Chaque fois que je vois mon copain au portail, je sais qu’on rentrera ensemble.
• Quand je vois mon copain au portail, je sais que nous rentrerons ensemble.
• Dès que je vois mon copain au portail, je sais que nous rentrerons ensemble.
  This one changes the meaning slightly to as soon as.

The original sentence is already correct and natural. The alternatives mainly change the tone or the exact nuance.