When we put a verb after a preposition, we normally use an –ing form, not an infinitive.

• I am fond of watching movies. (NOT I am fond of to watch movies.)
• John was arrested for stealing a policeman’s helmet. (NOT … for to steal …)
• Can you talk without opening your mouth?
• I am thinking of writing a novel.
• You must abstain from talking to such people.
• We got the job finished by burning the midnight oil.
• We look forward to hearing from you.

To as a preposition

To can be an infinitive marker (e.g. to work, to laugh). It can also be a preposition. When to is a preposition, it is followed by either a noun or the –ing form of a verb, but not normally by the infinitive. Common expressions in which this happens are look forward to, object to, used to, prefer to, get round to, in addition to.

Compare:

• I look forward to his next visit. (noun)
• I look forward to hearing from you. (NOT I look forward to hear from you.)
• I prefer the country to the city. (noun)
• I prefer swimming to walking.
• I am used to waiting for buses. (NOT I am used to wait for buses.)
• They objected to our entering the room.
• I object to working on Sundays.