Appelle-moi tout de suite si le train est en retard.

Because appelle is the imperative form of appeler (to call). The imperative is used for commands, instructions, or urgent requests.

So Appelle-moi means Call me.

In French, the imperative normally leaves out the subject pronoun. So instead of saying Tu appelle..., French just says Appelle...

This is the same idea as English Call me, not You call me.

In an affirmative imperative (a positive command), object pronouns come after the verb, and me changes to moi.

So:

• Appelle-moi = Call me
• not Appelle-me

Compare that with a normal sentence:

• Tu me appelles would still be wrong for other reasons, but the object pronoun in ordinary word order would be me before the verb: Tu m’appelles.

A very useful contrast is:

• Appelle-moi = call me
• Ne m’appelle pas = don’t call me

In French, when a pronoun follows the verb in an affirmative imperative, it is joined to the verb with a hyphen.

So you write:

• Appelle-moi
• Donne-lui
• Dites-nous

The hyphen is standard spelling here.

Because the tu form of most -er verbs loses its final -s in the imperative.

Compare:

• Present tense: tu appelles
• Imperative: appelle

Other examples:

• tu parles → Parle !
• tu regardes → Regarde !

One famous exception pattern is before y or en, where the -s comes back for pronunciation:

• Vas-y
• Parles-en

But in Appelle-moi, you use appelle with no -s.

Tout de suite is a very common expression meaning right away, immediately, or straight away.

It is an idiomatic phrase, so it is best learned as a whole unit. In this sentence, it adds urgency:

• Appelle-moi tout de suite = Call me immediately

After si meaning if, French usually uses the present tense when talking about a real future possibility.

So French says:

• si le train est en retard

Even though in English the meaning is future-related, French does not normally use the future tense here.

This pattern is very common:

• Si tu viens, je pars.
• Si j’ai le temps, je t’appelle.
• Appelle-moi si le train est en retard.

Le train refers to a specific train that both people already know about from the situation or context.

French often uses the definite article when the thing is already understood:

• le train = the train we are talking about

If you said un train, it would sound more like a train, meaning any train or an unspecified one.

En retard is a fixed expression meaning late or delayed. It is used with être:

• être en retard = to be late

So:

• Le train est en retard = The train is late / delayed

This is the normal everyday way to say it. For trains, another very common alternative is:

• Le train a du retard = The train is delayed / The train has a delay

You might hear it sometimes, but en retard is much more natural in everyday French here.

So these are more idiomatic:

• Le train est en retard
• Le train a du retard

Le train est retardé sounds more technical or more like a passive construction, and it is less common in ordinary conversation.

You would use the vous imperative:

• Appelez-moi tout de suite si le train est en retard.

This can mean:

• speaking politely to one person
• speaking to several people

So:

• Appelle-moi... = informal singular
• Appelez-moi... = formal singular or plural

In the negative imperative, the pronoun goes before the verb again, and moi becomes me (or m’ before a vowel sound).

So:

• Appelle-moi = Call me
• Ne m’appelle pas = Don’t call me

That means the pattern changes:

• positive command: verb + pronoun
• negative command: ne + pronoun + verb + pas

This is a very important contrast to remember.