Was de jarige maar nog niet hier!

Because this is an exclamatory wish construction in Dutch.
The pattern was ... maar ... means something like if only ... were ... or I wish ... were ....

So:

• Was de jarige maar nog niet hier! = If only the birthday person weren’t here yet!

This is why the verb comes first. It is not a normal statement like:

• De jarige was nog niet hier. = The birthday person was not here yet.

That version is just a past-tense statement, not a wish.

Formally, yes: was is the singular past tense of zijn.
But in this sentence it is not mainly expressing past time.

Instead, Dutch uses the past form here to express an unreal wish, much like English uses were in If only he were here.

So in this sentence, was does not mean the speaker is talking about the past. It means the speaker wishes the situation were different now.

De jarige means the birthday person, the one whose birthday it is, or the celebrant.

It comes from the adjective jarig, which means having one’s birthday.
Dutch can turn adjectives into nouns by using de or het, so:

• de jarige = the person whose birthday it is

This is very common in Dutch. It is gender-neutral and can refer to a child or an adult.

Because Dutch often uses adjectives as nouns when the context makes the person or thing clear. This is called a nominalized adjective.

So:

• jarig = having a birthday
• de jarige = the birthday person

English does this sometimes too, in expressions like the rich or the poor, but Dutch does it more freely.

Correct: here maar does not mean but.

In this kind of sentence, maar helps express a wish, often with frustration, longing, or regret. In English, there is no single direct word-for-word equivalent, but the whole pattern was ... maar ... corresponds to if only ... or I wish ....

So you should understand maar here as part of the wish construction, not as the usual conjunction but.

Nog niet means not yet.

That is important because it suggests the speaker expects the situation to change later. So the idea is not simply the birthday person is not here, but:

• the birthday person is not here yet
• they may arrive later

In this sentence, the speaker is wishing for that situation to still be true.

Because hier is the place element, and in Dutch it is very natural for that kind of information to come near the end of the clause.

Also, nog niet forms a tight unit meaning not yet, so it stays together before hier:

• nog niet hier = not here yet

That word order sounds natural and neutral in Dutch.

No word is missing. Was de jarige maar ... is already a complete and idiomatic Dutch wish sentence.

Dutch often uses verb-first word order for emotional wishes like this.
So the sentence is complete as it stands.

A different way to express a similar idea would be:

• Ik wou dat de jarige nog niet hier was. = I wish the birthday person wasn’t here yet.

That version is more explicit, but the original sentence is perfectly valid.

Yes. The exclamation mark fits the tone well.

This structure usually sounds more emotional, immediate, or expressive than a neutral sentence. It often conveys frustration, stress, or strong wishing.

So Was de jarige maar nog niet hier! sounds like the speaker really wishes the birthday person had not arrived yet.

Yes, that is the usual implication.

Because this is an unreal wish, the speaker is normally wishing for the opposite of the current situation. So the likely meaning is:

• the birthday person is already here
• the speaker wishes that were not the case yet

That is why nog niet is important: the speaker wishes the arrival had not happened yet.