BBC Learning English message boards

My question:
when should i put V-ing after "to"??

Teacher's answer:
ook forward to?is an expression and o?is a preposition. A preposition cannot be followed by a verb. A preposition is followed by a noun/pronoun/gerund.

eeing?= gerund.

AL

Reflection:
Frankly speaking, i didn't expect such short answer, and what i really wanted to know is what kind of situation should i put V-ing after "TO". The teacher just answered my examples and didn't give more example or details. Also, the anwser inclueded some illegible characters so i couldn't understand clearly. But over all, the response is fast and becasue they would filter the questions first, i am confident about their answers.