Ich schnalle mich im Auto an.

Because anschnallen is a separable verb.

• The full infinitive is sich anschnallen.

• In a normal present-tense main clause, the stem goes in the usual verb position, and the prefix moves to the end:

• Ich schnalle mich an.
• infinitive: mich anschnallen

So in Ich schnalle mich im Auto an, the an is the separated prefix.

Because sich anschnallen is usually used reflexively when you mean that you buckle yourself up.

• ich → mich
• du → dich
• er/sie/es → sich

So:

• Ich schnalle mich an = I buckle myself up.
• Du schnallst dich an = You buckle yourself up.

In English, myself is often optional, but in German the reflexive pronoun is normally included here.

Because the reflexive pronoun here is in the accusative.

The verb pattern is basically:

• sich anschnallen

That means the person being buckled in is the direct object, so German uses the accusative reflexive pronoun:

• ich → mich
• du → dich

You would use mir only in verbs or structures that require the dative, which this one does not in this sentence.

im is a contraction of in dem.

So:

• im Auto = in dem Auto

It uses the dative case here because it describes a location: where the action happens.

• Wo? — im Auto = in the car

If it were about movement into the car, German would normally use the accusative:

• ins Auto = into the car

But here the person is already in the car, so im Auto is correct.

Because an belongs to the separable verb and must go to the end, while im Auto sits in the middle part of the sentence.

German main-clause order here is roughly:

• Ich = subject
• schnalle = conjugated verb
• mich = reflexive pronoun
• im Auto = location
• an = separated prefix

So the sentence structure is completely normal.

Yes. That is a very common sentence.

• Ich schnalle mich an. = I’m buckling up.

Adding im Auto just gives extra information about where.

In real life, if the context already makes it clear that you are in a car, people often leave im Auto out.

Yes, but it is slightly different in focus.

• Ich schnalle mich an. = I buckle myself up.
• Ich schnalle den Gurt an. = I fasten the seat belt.

Both can work in context, but sich anschnallen is very common when talking about seat belts and buckling up in general.

You may also hear:

• Ich lege den Sicherheitsgurt an.
• Ich mache den Gurt fest.

But Ich schnalle mich an is very natural everyday German.

On its own, schnallen can mean something like to fasten with a buckle. It is related to die Schnalle, meaning buckle.

With the prefix an-, anschnallen becomes the standard verb for buckling up or fastening a seat belt.

So this sentence is built from a very literal idea: fastening yourself with a buckle.

Usually, yes—especially in modern everyday German.

If someone says:

• Bitte anschnallen!

the normal meaning is:

• Please fasten your seat belt / Please buckle up!

So even though the verb is literally about fastening with a buckle, in everyday usage it strongly suggests seat belts unless the context says otherwise.

Yes, but the meaning stays basically the same while the emphasis changes.

For example:

• Ich schnalle mich im Auto an.
• Im Auto schnalle ich mich an.

Both are grammatical.

The first is the most neutral if you are simply stating the action. The second puts more emphasis on im Auto.

What does not change in a main clause is that the conjugated verb stays in second position and the separable prefix an stays at the end.

With a modal verb, the separable verb stays together at the end as an infinitive:

• Ich muss mich im Auto anschnallen.
• Ich will mich im Auto anschnallen.

Notice the difference:

• main clause without modal: Ich schnalle mich im Auto an.
• with modal: ... mich im Auto anschnallen

So the prefix an only separates in certain finite verb forms, not when the verb is left as an infinitive.

Yes, it is natural and correct.

That said, in many everyday situations Germans would probably say the shorter version:

• Ich schnalle mich an.

because the car context is often obvious.

Still, Ich schnalle mich im Auto an is perfectly fine if you want to explicitly mention the location.