Je vais retourner au supermarché si les concombres sont encore en promotion.

Both are possible, but they are not exactly the same.

• je vais retourner = the near future
  This often sounds a bit more immediate or connected to a current plan.
• je retournerai = the simple future
  This can sound a little more neutral, formal, or less immediate.

So Je vais retourner au supermarché... suggests something like I’m going to go back to the supermarket..., with a sense that the speaker is considering doing it soon.

Here, retourner means to go back / to return.

In this sentence, it means the speaker will go back to the supermarket.

Be careful, because retourner can also mean other things in different contexts, such as:

• to return something
• to turn over
• to flip

But in retourner au supermarché, the meaning is clearly to go back to the supermarket.

Because in French, à + le contracts to au.

So:

• à + le = au
• à + les = aux

That means:

• au supermarché = to the supermarket

You cannot normally say à le supermarché in standard French.

This is a very common French pattern.

After si meaning if, French does not normally use the future tense when talking about a real possible condition. It uses the present tense.

So French says:

• si les concombres sont encore en promotion
  literally: if the cucumbers are still on sale

Even though English often uses future meaning in the main idea, French keeps the verb after si in the present here.

A useful pattern is:

• si + present, then future / near future / imperative in the other clause

Examples:

• Si j’ai le temps, je viendrai.
• Si tu veux, on va partir.

So this sentence is perfectly normal.

Here, encore means still.

So:

• sont encore en promotion = are still on sale

This means the promotion is already happening now, and the speaker is wondering whether it continues.

Encore can also mean again in other contexts, so learners often need to check the situation carefully.

Examples:

• Il pleut encore. = It’s still raining.
• Dis-le encore. = Say it again.

En promotion means on sale, on special offer, or discounted.

This is a fixed French expression. You usually just learn it as a chunk:

• être en promotion = to be on sale

French often uses en in set expressions like this.

Other similar expressions:

• en retard = late
• en colère = angry
• en vacances = on vacation

So it is best to memorize en promotion as the normal way to say on sale.

In this sentence, les concombres refers to a specific set of cucumbers the speaker has in mind — most likely the cucumbers at that supermarket or the cucumbers that are on promotion.

French often uses the definite article more broadly than English does.

So les concombres here can feel like:

• the cucumbers
• or more naturally in English, simply cucumbers, depending on context

If you said des concombres, that would usually suggest some cucumbers, which is a different idea.

Yes. That would also be correct.

The difference is mainly one of nuance:

• Je vais retourner au supermarché...
  sounds like a current intention or likely next step
• Je retournerai au supermarché...
  sounds more like a straightforward future statement

In everyday spoken French, the near future with aller + infinitive is extremely common.

No. French si has two different uses.

In this sentence, si means if:

• si les concombres sont encore en promotion = if the cucumbers are still on sale

But si can also mean yes in a special situation: when contradicting a negative statement.

Example:

• Tu n’as pas faim.
• Si, j’ai faim.
  = Yes, I am hungry.

So the meaning depends on context.

Yes, there is usually a liaison.

• les normally ends with a silent s
• but before a vowel sound, that s is pronounced like z

Since concombres begins with a vowel sound, les concombres is pronounced roughly like:

• lay zon-combr

So you hear:

• les concombres → lez concombres

This liaison is normal and expected in careful everyday speech.

Because the idea of back is already included in retourner.

So:

• retourner au supermarché = to go back to the supermarket

French often expresses an idea with one verb where English uses a verb plus an extra word.

That is why you do not need a separate word for back here.