Si le garagiste termine plus tôt, le mécanicien nous appellera avant midi.

Because after si meaning if, French normally uses the present tense for a real or likely future condition.

So French says:

• Si le garagiste termine plus tôt...
• literally: If the mechanic finishes earlier...

Then the main clause can use the future:

• ...le mécanicien nous appellera avant midi.

This pattern is very common:

• Si + present, future
• Si tu viens, je partirai.
• If you come, I’ll leave.

A common learner mistake is using the future after si in this kind of sentence.

Because the phone call has not happened yet. The sentence is talking about what will happen if the condition is met.

• appellera = will call

So the structure is:

• If X happens, Y will happen
• Si + present, future

That is why the sentence has:

• termine = present
• appellera = future

They are related jobs, but not always exactly the same.

• un garagiste usually means a garage owner, garage worker, or someone associated with a repair shop
• un mécanicien means a mechanic, the person who does mechanical work

In everyday use, there can be overlap, but the sentence treats them as two different people:

• the garagiste finishes earlier
• the mécanicien will call us

So you should not assume they are automatically the same person.

Plus tôt means earlier.

In this sentence:

• termine plus tôt = finishes earlier

It often refers to earlier than expected, earlier than usual, or earlier than some previously mentioned time.

Compare:

• plus tôt = earlier
• plus tard = later

Examples:

• Je partirai plus tôt. = I’ll leave earlier.
• On verra ça plus tard. = We’ll deal with that later.

Because nous is an object pronoun, and in French object pronouns usually go before the conjugated verb.

So:

• le mécanicien nous appellera = the mechanic will call us

Not:

• le mécanicien appellera nous ✘

This is a basic French word-order rule.

Other examples:

• Il me voit. = He sees me.
• Elle vous écrira. = She will write to you.
• Ils nous aideront. = They will help us.

Because French usually does not use an article in this expression.

• avant midi = before noon

This is the standard fixed phrase.

Similarly:

• après midi is not used in the same way here; instead French says cet après-midi for this afternoon
• avant minuit = before midnight

So avant midi is just the normal idiomatic form.

Yes. French can put the si-clause first or second.

So you can say:

• Si le garagiste termine plus tôt, le mécanicien nous appellera avant midi.
• Le mécanicien nous appellera avant midi si le garagiste termine plus tôt.

Both are correct.

The version with si first often sounds a little more natural when introducing the condition first, but both are common.

Also note the punctuation:

• When the si-clause comes first, a comma is often used.
• When it comes second, the comma is often omitted.

Yes, finir could work in many contexts, but terminer is perfectly natural here.

Compare:

• terminer = to finish, to complete, to end
• finir = to finish, to end

In many situations they overlap:

• Si le garagiste finit plus tôt...
• Si le garagiste termine plus tôt...

Both can sound natural.

Sometimes terminer can feel slightly more focused on completing a task, while finir is often more general, but the difference is not huge here.

Because here the sentence is talking about specific people:

• le garagiste = the garage worker / the garage owner
• le mécanicien = the mechanic

These are the subjects of the sentence, so French uses the definite article just as English uses the.

This is different from sentences like:

• Il est mécanicien. = He is a mechanic.

After être plus a profession, French often does not use an article.

But here we are not naming someone’s profession after être. We are identifying specific people in the situation, so le is normal.

Yes, it is a conditional idea in the everyday sense: one thing happens if another thing happens.

But in French grammar, this pattern does not require the conditional mood.

French uses:

• si + present
• future in the main clause

So:

• Si le garagiste termine plus tôt, le mécanicien nous appellera avant midi.

The conditional tense would appear in different patterns, for example:

• Si le garagiste terminait plus tôt, le mécanicien nous appellerait avant midi.
• If the garage worker finished earlier, the mechanic would call us before noon.

That version sounds more hypothetical or less likely.

So the original sentence is the normal real future possibility pattern.

appellera comes from the verb appeler.

The double l is part of the verb’s spelling pattern. In many forms of appeler, the l doubles before a silent ending:

• j’appelle
• tu appelles
• il appelle
• ils appellent
• j’appellerai
• il appellera

The pronunciation is roughly:

• appellera ≈ ah-pehl-ruh

A few useful notes:

• the final -a is pronounced
• the r is the French r
• the stress is not as strong as in English

If you know the infinitive appeler, the future stem appeller- is something worth memorizing.