Enkai is my best friend.

It's difficult to make sentences using interrogatives.

It's difficult to make sentences using infinitives.

The interjection 'Ah!' expresses his surprise.

He expresses numbers using numerals.

Yushi is my best friend.

That person is my friend.

He is very careful in his choice of diction.

He is very careful in choosing words and phrases.

A determiner plays a role in specifying a noun.