Pas op, het stopcontact vonkt!

Because the full verb is oppassen, which is a separable verb in Dutch.

• Infinitive: oppassen
• Imperative (command) to one person: pas op!

In separable verbs, the prefix often moves to the end in a main clause or command. So:

• Ik pas op. = I watch out / I’m careful.
• Pas op! = Watch out!

This is very common in Dutch.

It is both:

1. the imperative of oppassen
2. a very common warning expression, like watch out! or be careful!

So in this sentence, Pas op functions as a direct warning.

You will often hear it by itself:

• Pas op!
• Pas op voor de hond! = Watch out for the dog!

Because it is a command. In Dutch, as in English, commands usually leave out the subject.

Compare:

• Jij past op. = You are being careful.
• Pas op! = Be careful! / Watch out!

The hidden subject is basically you.

Het is the definite article for neuter nouns in Dutch.
Stopcontact is a het-word, so it takes het:

• het stopcontact = the socket / outlet

This is something you generally have to learn with the noun. Not all nouns take de.

It is a compound noun, which is very common in Dutch. Dutch often joins nouns into one word.

• stopcontact = electrical socket / outlet

Even if the parts seem transparent, Dutch normally writes this as one word, not separate words.

This is a pattern you will see everywhere in Dutch.

Because vonkt is the present tense, third-person singular form of the verb vonken.

Conjugation:

• ik vonk
• jij vonkt
• het vonkt
• wij vonken

Since het stopcontact is singular, the verb must also be singular:

• het stopcontact vonkt

Here, vonken means to spark or to give off sparks.

So het stopcontact vonkt means the socket is producing sparks. In English, you might also naturally say:

• The outlet is sparking.

Dutch often uses the simple present for this kind of immediate situation.

The comma separates the warning from the explanation:

• Pas op, het stopcontact vonkt!

It works a bit like:

• Watch out, the outlet is sparking!

The first part is an exclamation or warning, and the second part tells you why. You could also split it into two sentences:

• Pas op! Het stopcontact vonkt!

Both are natural.

Yes, that is possible, but it means something slightly different in structure.

• Pas op voor het stopcontact. = Watch out for the socket.
• Pas op, het stopcontact vonkt! = Watch out, the socket is sparking!

The original sentence is more informative and urgent because it tells you exactly what is wrong.

Not exactly.

• Pas op! usually means watch out! / be careful! and often warns about danger.
• Let op! usually means pay attention!

So in this sentence, pas op is the better choice because an outlet sparking is dangerous.

A few useful pronunciation points:

• stopcontact: the stress is on the first part: STOP-contact
• vonkt: the o is short, and the final kt is clearly pronounced

Also:

• op has a short vowel, like op in a clipped sound
• het is often pronounced weakly in everyday speech, almost like ut or a very short hət

You do not need a perfect accent to be understood, but keeping the final consonants clear in vonkt helps.

Yes, very natural. It sounds like something someone would say in an urgent situation.

It is short, direct, and idiomatic:

• Pas op = immediate warning
• het stopcontact vonkt = clear explanation of the danger

That makes it a very useful real-world Dutch sentence.