Ég held að líf hans sé í hættu.

I think his life is in danger.Why is the hættu used here instead of hætta? Is it a mistake or some turn of phrase?

Hætta is a weak feminine noun, and therefore the a ending changes to u in all cases in singular other than nominative.

Ahh, thank you. I’m still trying to find the best way to teach myself the grammar. Clozemaster’s Icelandic course is pretty limited at the moment.

Icelandic grammar is stupidly complicated. There’s a new website called icelandicgrammar.com that does a good job of breaking a lot of it down.