Hi

Please check the following sentences:
1-Do you enjoy hiking?
why do we use ing for hiking?

2-Many people like my plan. Will you agree to it?
Many people like my plan. Will you agree with it?
which is right?

3-If you don't understand, don't hesitate to ask a question
If you don't understand, don't hesitate asking a question
which is right?

4-Tomorrow is Mary's birthday. Let's it congratulate it
Tomorrow is Mary's birthday. Let's it celebrate it
which is right?