Infinitive

Forming the infinitive:

the to-infinitive = to + base (to sit)
the zero infinitive = base (sit)

Functions of the to-infinitive:

1. To indicate the purpose or intention of an action. eg: She came to collect her pay cheque.
2. As the subject of the sentence: eg: To visit the Grand Canyon is my life-long dream.
3. To indicate what something can or will be used for: eg: I don't have anything to wear.
4. After adjectives: subject + to be + adjective + (for/of someone) + to-infinitive + (rest of sentence). eg: It is good to talk.
5. To make a comment or judgement: Subject + to be + noun phrase + to-infinitive. eg: It was a stupid to place to stay.
6. With adverbs. eg: I had too many books to carry.
7. With question words: The verbs ask, decide, explain, forget, know, show, tell, & understand can be followed by a question word such as where, how, what, who, & when + the to-infinitive. eg: Do you understand what to do?

Functions of the zero infinitive:

1. After auxiliaries. eg: She can't speak to you.
2. After verbs of perception. eg: We heard them close the door.
3. After the verbs 'make' and 'let'. eg: Her parents let her stay out late.
4. After the expression 'had better'. eg: We had better reserve a room in the hotel.
5. with "why". eg: Why wait until tomorrow?

Gerund

1. The gerund as the subject of the sentence: Flying makes me nervous.
2. The gerund as the complement of the verb 'to be': One of his duties is attending meetings.
3. The gerund after prepositions: She is good at painting.
4. The gerund after phrasal verbs: He kept on asking for money.
5. The gerund in compound nouns: They have a swimming pool in their back yard. 
6. The gerund after some expressions: I can't stand being stuck in traffic jams.