A Gerund is a verbal noun - it is a verb acting as a noun. Gerunds are made by adding -ing to the base verb e.g. watch / watching.
Gerunds can either be the subject or object of a verb:
Gerund as a subject: "Skiing is great."
Gerund as an object: "I love skiing."
Gerunds are often used after state verbs e.g.
"I enjoy walking in the park."
"She hates smoking."
When you use a verb after a preposition in a sentence, use a gerund.
"He ended his speech by thanking everyone."
"Don't cross the road without looking."
We also use gerunds after two-word prepositions.
"I'm tired of working on the weekend."
"She dreams of winning a gold medal."
Infinitives are the base form of the verb e.g. look, see, watch. Infinitives can either be used alone or with to.
Use an infinitive verb (without to) with modal verbs
"We could go shopping tomorrow."
"You had better call Jane before it gets too late."
We can use to + infinitive as the object of verbs like want and wish.
"I really want to watch it again."
"Do you wish to speak to her about it."
We use to + infinitive after an adjective / adverb + enough.
"Simon isn't old enough to drive."
"You are too short to ride on the roller-coaster."