Formal or Informal Expressions

The theme of an English workshop which I attended the other day was "Conversations in the Office."
One of topics is "Formal or Informal expressions."

Some expressions that I thought is formal were informal, and vice versa. It's quite difficult.

The followings are some of the expressions which I thought are formal but actually informal.

make a plan

Police are looking into the crime.
I think you should try this restaurant.
turn in a report
Excuse me, do you have a few minutes?

But polite expressions in Japanese are MUCH more difficult. I'm sure there are many learners of Japanese language struggle with Japanese polite expressions. It might be good that I'm a learner of English, not Japanese. haha.