Na een week was de griep voorbij.

Because Dutch often puts a time expression at the beginning of the sentence if that is the context you want to highlight.

So:

• Na een week was de griep voorbij.
• De griep was na een week voorbij.

Both are correct. The first one emphasizes the time frame a bit more: after a week.

This is because of the Dutch verb-second rule.

In a main clause, the finite verb normally comes in the second position. If you put Na een week first, that counts as the first element, so the verb was must come next:

• Na een week = first element
• was = second element
• de griep = subject

This is called inversion. It is very common in Dutch.

Compare:

• De griep was voorbij.
• Na een week was de griep voorbij.

Na means after.

It is a preposition, and here it introduces a time phrase:

• na een week = after a week

You can use na with many other time expressions too:

• na een dag = after a day
• na een paar uur = after a few hours
• na het eten = after dinner / after eating

Een without an accent usually means a/an.

So na een week means after a week.

If you write één, with an accent, it emphasizes the number one:

• na een week = after a week
• na één week = after exactly one week / after one single week

In this sentence, the normal form is een week.

In Dutch, nouns usually take an article, and griep is a de-word:

• de griep = the flu

Unlike English, where illnesses sometimes appear without an article, Dutch often uses the article with disease names in this kind of sentence.

You would learn griep together with its article: de griep.

Here voorbij means over, finished, or gone.

In was ... voorbij, it functions as a complement with zijn:

• De film is voorbij. = The film is over.
• De pijn was voorbij. = The pain was gone.
• De griep was voorbij. = The flu was over.

So voorbij zijn is a very common Dutch way to say that something has ended.

Yes, very much so.

Dutch often uses zijn + voorbij to mean that something has come to an end:

• Het feest is voorbij. = The party is over.
• De storm was voorbij. = The storm had passed / was over.
• De griep was voorbij. = The flu was over.

So learners should remember voorbij zijn as a common pattern.

Was is the simple past of zijn.

Dutch often uses the simple past for storytelling, description, and past situations, especially in written language and in neutral narration.

So:

• Na een week was de griep voorbij.

is a natural way to describe a past situation.

A present perfect version is also possible in some contexts:

• Na een week is de griep voorbij geweest.

But that sounds less natural here. The simple past is the normal choice.

Yes. That is completely correct.

Compare:

• Na een week was de griep voorbij.
• De griep was na een week voorbij.

The meaning is basically the same. The difference is mostly emphasis and information structure:

• Na een week... puts the time frame first.
• De griep... starts with the subject.

In this sentence, it can match either depending on how you translate it:

• The flu was over
• The flu was gone

Dutch voorbij can cover both ideas: something has ended, passed, or is no longer there.

So the exact English wording may vary, but the Dutch structure stays the same: zijn + voorbij.

The original sentence is already natural. But in everyday speech, people might also say:

• Na een week was ik van de griep af. = After a week I was over the flu / rid of the flu.
• De griep was na een week over.
• Na een week was het voorbij.

Your original sentence is clear, standard, and idiomatic. It is a very good model sentence for learners.