In normal, conversational English, the word "yet" comes after the verb:

He hasn't started yet.

But you can also put "yet" before the verb. Doing this makes the sentence seem a lot more formal:

It has not yet begun.

There's a famous example of this phrase from history. In a battle during the Revolutionary War between America and Britain, a captain in the American navy was told to surrender by the British. He replied:

I have not yet begun to fight!

This means "I haven't even started fighting yet!" The captain was telling the British soldiers that he would not give up.

Here's another example. In a nature film about a family of bears, the narrator might say:

The cubs have not yet reached maturity, so they continue to stay very close to their mother.

Announcers and narrators sometimes use formal English like this.