(something) has not yet (done something)

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.

Police have (done something)

On television news shows, the news anchors describe what the police have done beginning with "Police have...":

Police have launched an investigation into the cause of the fire.

In most other situations, we say "The police...":

The police are trying to figure out what caused the fire.

"Police have..." sounds more formal.

determine (something)

To "determine" something means to figure it out or make a decision about it. "Determine" is a formal word, so you can use it in situations like when describing legal decisions:

The judge determined that Medco was responsible for the patients' medical issues. They were required to pay a large settlement.

Or you can use it in a business setting:

Have they determined when the merger will take place?

After "determined", you can use a clause like in the previous examples, or a noun:

Once we've determined the cause of the outage, we'll send a full report.

the cause of death

"Cause of death" is a phrase used by police, doctors, and the government. It means the official reason that someone died. This reason is printed on the dead person's death certificate.

A person's "cause of death" is written by doctors, so it includes a lot of detailed medical terminology. But when people discuss someone's "cause of death" in normal conversation, we talk about things like:

• heart attack
• head trauma (getting hit in the head)
• murder
• suicide