Wenn wir in Not sind, rufen wir die Polizei.

In this sentence, wenn can be understood as when in the sense of whenever. It introduces a general condition:

Wenn wir in Not sind, rufen wir die Polizei.
= When/Whenever we are in trouble, we call the police.

In many contexts, wenn can also mean if, especially when the condition is open or uncertain. Here, because the sentence describes a general rule, when/whenever is the most natural English interpretation.

Because the first part is a subordinate clause introduced by wenn. In German, the conjugated verb usually goes to the end of a subordinate clause.

So:

• wir = subject
• in Not = expression
• sind = verb, placed at the end

That is why German says:

Wenn wir in Not sind

and not Wenn wir sind in Not.

German main clauses follow the verb-second rule. That means the conjugated verb must come in the second position.

Since the sentence begins with the subordinate clause Wenn wir in Not sind, that whole clause counts as position 1. So the main clause must begin with the verb:

Wenn wir in Not sind, rufen wir die Polizei.

Structure:

1. Wenn wir in Not sind
2. rufen
3. wir
4. die Polizei

If the main clause stood alone, you would normally say:

Wir rufen die Polizei.

German normally requires each clause to have its own subject if needed. The first wir belongs to the wenn clause, and the second wir belongs to the main clause.

So the structure is:

• Wenn wir in Not sind = subordinate clause
• rufen wir die Polizei = main clause

Even though English also repeats the subject here, learners sometimes expect German to leave it out. Usually, it does not.

In Not is an idiomatic expression meaning something like:

• in trouble
• in distress
• in need
• in an emergency depending on context

In this sentence, it means that we are in a situation where we need help.

So Not here is not the English word not. It is a German noun: die Not, meaning distress, hardship, or need.

In Not sein is a fixed expression, so German commonly says:

in Not sein = to be in distress / in trouble

Without an article, it sounds idiomatic and general.

In der Not also exists, but it usually has a different feel, more like in the time of need or in an emergency situation, and often appears in set phrases such as In der Not frisst der Teufel Fliegen. In your sentence, in Not sein is the normal wording.

Here, Not is in the dative.

The preposition in can take either:

• accusative for movement into something
• dative for location or state

Since sein describes a state rather than movement, German uses the dative idea here:

in Not sein

This expression is somewhat fixed, so learners usually just memorize in Not sein as a chunk.

In German, many nouns are used with an article where English often leaves it out.

So German says:

• die Polizei rufen = to call the police
• die Schule = school in some contexts
• die Natur = nature

English often says call police only in certain special styles, but normal English is also usually call the police. So here German and English are actually quite similar.

German often uses the present tense to express general truths, habits, or standard actions.

So:

Wenn wir in Not sind, rufen wir die Polizei.

means something like:

• When we are in trouble, we call the police.
• If we are in trouble, we call the police.

It describes what people generally do in that situation. You do not need a future form here.

Yes, very often German uses die Polizei anrufen, which means to phone the police.

There is a small difference:

• die Polizei rufen can mean call for the police more generally
• die Polizei anrufen specifically means call the police by phone

So both are possible, but anrufen is more specific.

Example:

• Wenn wir in Not sind, rufen wir die Polizei.
• Wenn wir in Not sind, rufen wir die Polizei an.

Notice that with anrufen, the prefix an- moves to the end in a main clause.

Yes. In German, a comma is normally required before a subordinate clause or between a subordinate clause and the main clause.

So this is correct:

Wenn wir in Not sind, rufen wir die Polizei.

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

Yes. You can also put the main clause first:

Wir rufen die Polizei, wenn wir in Not sind.

This means the same thing.

The difference is mainly one of emphasis and style:

• Wenn wir in Not sind, rufen wir die Polizei.
  The condition comes first.
• Wir rufen die Polizei, wenn wir in Not sind.
  The action comes first.

In both versions, the wenn clause still sends its verb to the end.