After a preposition we cannot put a to-infinitive. Instead, we use an –ing form.

I am confident of winning. (NOT I am confident of to win.) (NOT I am confident to win.)

I am thinking of writing a novel. (NOT I am thinking of to write a novel.) (NOT I am thinking to write a novel.)

Always check the oil before starting the car. (NOT Always check the oil before to start the car.)

We finished the job by working twenty hours a day. (NOT We finished the job by to work twenty hours a day.)

He mowed the lawn instead of watching TV. (NOT He mowed the lawn instead of to watch TV.)

I look forward to seeing you. (I look forward to see you.)

Note that after some verbs we use for + object + infinitive. An –ing form is not possible in this case.

We are still waiting for them to arrive. (NOT We are still waiting for them arriving.)

Can you arrange for us to get tickets for the concert? (NOT Can you arrange for us getting tickets for the concert?)

To as a preposition

To is an infinitive marker. It can also be used as a preposition. When ‘to’ is used as a preposition, it should be followed by an –ing form or a noun.

I look forward to your next letter.

I look forward to hearing from you. (NOT I look forward to hear from you.)