Bitte ruf mich vorher an, wenn du kommst.

Because the verb is anrufen, which is a separable verb.

In many German sentences, separable verbs split into:

• the main verb part in its normal position
• the prefix at the end

So:

• infinitive: anrufen
• imperative here: ruf ... an

That is why the sentence says:

• Ruf mich an. = Call me.

and not anruf mich.

Again, this happens because anrufen is separable.

In a main clause, the prefix an- moves to the end:

• Ich rufe dich an.
• Ruf mich an.

But when the verb stays in the infinitive, it does not split:

• Ich will dich anrufen.
• Kannst du mich anrufen?

So in Bitte ruf mich vorher an, an is at the end because this is a main clause with a conjugated separable verb.

Because this is the informal singular imperative: a command to one person you call du.

The verb rufen has:

• du rufst = you call
• imperative: ruf! = call!

So:

• Du rufst mich an. = You call me.
• Ruf mich an! = Call me!

German often forms the du imperative by using the verb stem alone.

In German, the du is usually left out in the imperative.

So instead of saying:

• Du ruf mich an

German normally says:

• Ruf mich an!

This is just how commands are usually formed. The subject is understood.

Compare:

• Komm! = Come!
• Warte! = Wait!
• Ruf mich an! = Call me!

Bitte makes the command sound more polite.

Compare:

• Ruf mich vorher an. = more direct
• Bitte ruf mich vorher an. = more polite, softer

In English, this works a lot like please.

Its position is flexible. You could also hear:

• Ruf mich bitte vorher an.

That means basically the same thing.

Because anrufen takes a direct object in the accusative case.

So:

• ich = I
• mich = me
• mir = to me / for me

With anrufen, German uses the accusative, so:

• Ruf mich an. = Call me.
• Er ruft mich an. = He is calling me.

Not:

• ruf ich an
• ruf mir an

Vorher means beforehand, in advance, or before that.

In this sentence, it tells you when to call:

• first call
• then come

So the idea is:

• Call me beforehand when you’re coming.

It often appears in sentences like:

• Sag mir vorher Bescheid. = Let me know beforehand.
• Ich muss vorher noch einkaufen. = I still have to shop first / beforehand.

Wenn means when here, introducing a subordinate clause.

So:

• wenn du kommst = when you come / when you’re coming

This clause gives the condition or time relation for the main clause:

• Bitte ruf mich vorher an = please call me beforehand
• wenn du kommst = when you come

Together:

• Please call me beforehand when you come.

Because wenn introduces a subordinate clause, and in German subordinate clauses the conjugated verb usually goes to the end.

Compare:

Main clause:

• Du kommst.

Subordinate clause:

• ..., wenn du kommst.

This is a very important German word-order rule.

Other examples:

• Ich bleibe zu Hause, weil ich krank bin.
• Sag Bescheid, wenn du Zeit hast.

In both cases, the conjugated verb comes at the end of the subordinate clause.

Yes. You can also say:

• Wenn du kommst, ruf mich bitte vorher an.

That is completely natural.

When the subordinate clause comes first, the main clause still follows normal German word-order rules, which means the verb comes right after the clause:

• Wenn du kommst, ruf ... an

You do not say:

• Wenn du kommst, du rufst ...

Not always. German has more than one way to say when.

Here, wenn is correct because it refers to something repeated, general, or in the future:

• wenn du kommst = when you come / whenever you come / when you’re coming

But for a single event in the past, German often uses als:

• Als du kamst, war ich müde. = When you came, I was tired.

So in your sentence, wenn is the natural choice.

It is informal, because it uses du and the du-imperative ruf.

If you wanted to say it formally to Sie, you would say:

• Bitte rufen Sie mich vorher an, wenn Sie kommen.

So:

• ruf = informal, one person
• rufen Sie = formal

Yes, usually anrufen means to call someone on the phone.

That is important because English call can sometimes be broader, but German anrufen is specifically about telephoning.

Examples:

• Ich rufe dich morgen an. = I’ll call you tomorrow.
• Hat sie schon angerufen? = Has she called already?

So in this sentence, it clearly means:

• please phone me beforehand

Yes, German word order is somewhat flexible here.

These are all natural:

• Bitte ruf mich vorher an.
• Ruf mich bitte vorher an.
• Ruf mich vorher bitte an.
  This one is possible, though a bit less neutral in everyday speech.

The most common versions are probably:

• Bitte ruf mich vorher an.
• Ruf mich bitte vorher an.

The basic meaning stays the same. The differences are mostly about emphasis and style.