1.Mary has difficulty in studying English grammar.
= Mary has trouble (in) studying English grammar.
= Mary has difficulty/trouble with English grammar.
2.Why not take a walk after dinner?
= What/How about taking/having a walk after dinner?
3.I am afraid to talk in front of people.
= I am afraid of talking in front of people.
注:/ 表示可以任意选用其一;( )表示其内部分可以省略不写.