“当” is a verb that means “act as”， “work as”. It is followed by a noun of profession or post.
What do you want to be when you grow up?
“极了”in the structure of “adj.+极了” means “a great degree ”.
适应 accustom to
“适应” is a verb that means “adapt to (a condition or a need)” .
The North Pole is very cold. If I could eat one bucke of ice-cream every day, I would be able to adapt to the weather there.