Als de film begint, zullen wij waarschijnlijk nog niet thuis zijn, maar we zullen wel gegeten hebben.

Here als means when.

Dutch als can mean either if or when, depending on context. In this sentence, the film starting is treated as an expected event, so the natural meaning is when the film starts.

You could also say wanneer de film begint, but als de film begint is very common and natural.

After words like als, wanneer, zodra, and similar time-linking words, Dutch usually uses the present tense to talk about the future.

So:

• Als de film begint = When the film starts
• not normally Als de film zal beginnen

This is different from English, which can sometimes feel more explicitly future-oriented.

Because als de film begint is a subordinate clause.

In Dutch subordinate clauses, the finite verb usually goes to the end. So the order is:

• als
  • subject + rest + verb
• Als de film begint

That is a very common Dutch pattern.

Because the sentence begins with a subordinate clause: Als de film begint.

When a Dutch main clause starts with something other than the subject, the finite verb must still stay in the second position. This causes inversion:

• Als de film begint, zullen wij ...
• not Als de film begint, wij zullen ...

This is one of the most important word-order rules in Dutch main clauses.

Both wij and we mean we.

The difference is mostly about style and emphasis:

• we is the normal unstressed form and is very common in speech
• wij is the fuller, stressed form and can sound a bit more formal or emphatic

In this sentence, mixing them is possible and natural enough. A speaker might say the whole sentence with we as well:

• Als de film begint, zullen we waarschijnlijk nog niet thuis zijn, maar we zullen wel gegeten hebben.

Nog niet means not yet.

So:

• nog niet thuis zijn = not be home yet

The word nog adds the idea that the situation may change later. Without nog, niet thuis zijn would simply mean not be at home, without the strong sense of yet.

Because thuis here is an adverb, not a noun.

Thuis zijn is a fixed and very common expression meaning to be at home. Dutch does not use an article here.

So:

• Ik ben thuis = I am at home
• Wij zijn nog niet thuis = We are not home yet

Here wel gives a positive contrast.

The first part says something negative:

• we will probably not be home yet

Then the second part contrasts with that:

• but we will have eaten

So wel is like saying however, at least, or adding stress to the positive idea. It does not mean English well here.

A natural way to feel it is:

• ..., but we will at least have eaten
• or ..., but we will have eaten, though

Because after zullen, Dutch uses the perfect infinitive to form the future perfect.

The structure is:

• zullen
  • past participle + hebben/zijn

So:

• zullen gegeten hebben = will have eaten

Compare:

• we hebben gegeten = we have eaten
  here hebben is the finite verb
• we zullen gegeten hebben = we will have eaten
  here gegeten hebben works together as the infinitive part

This is the normal Dutch pattern for the future perfect.

Because the verb eten forms its perfect tenses with hebben.

So:

• ik heb gegeten
• wij zullen gegeten hebben

Only some verbs take zijn, especially verbs of movement or change of state, such as:

• gaan → is gegaan
• komen → is gekomen
• worden → is geworden

But eten uses hebben.

It is the future perfect.

It describes something that will be completed before a point in the future. In this sentence, the future reference point is:

• Als de film begint = when the film starts

And the completed action before that point is:

• we zullen wel gegeten hebben = we will have eaten

So the sentence is saying that by the time the film starts, eating will already be finished.

Waarschijnlijk means probably, and in this sentence it modifies the whole idea of not being home yet.

The placement is very natural in Dutch:

• zullen wij waarschijnlijk nog niet thuis zijn

It comes after the finite verb zullen and before the rest of the predicate. That is a common place for adverbs like waarschijnlijk, misschien, natuurlijk, and zeker.

If the sentence started directly with the subject, you would normally get:

• Wij zullen waarschijnlijk nog niet thuis zijn.

Yes, it is standard to place a comma after an initial subordinate clause like Als de film begint.

So this punctuation is normal:

• Als de film begint, zullen wij ...

The comma helps show where the subordinate clause ends and the main clause begins.