1. Gerunds (verb + -ing)

Use gerunds in these situations:

• As the subject of a sentence
  Example: Swimming is good exercise.
• After certain verbs
  Examples: enjoy, finish, practice, suggest, avoid, consider, delay, mind, quit, risk
  Example: I enjoy cooking.
• After prepositions
  Example: He's interested in learning English.
• In certain fixed expressions
  Example: It's worth visiting that museum.

2. Infinitives (to + verb)

Use infinitives in these situations:

• To express purpose
  Example: I went to the store to buy milk.
• After certain adjectives
  Example: It's important to exercise regularly.
• After certain verbs
  Examples: want, decide, hope, plan, agree, expect, learn, offer, promise, refuse
  Example: They decided to go to the party.
• In certain fixed expressions
  Example: To be honest, I don't like that movie.

3. Base Form of the Verb (Infinitive without 'to')

Use the base form:

• After modal verbs
  Example: You should eat more vegetables.
• After 'let' and 'make' (in active voice)
  Example: Let me think about it.
• After perception verbs (see, hear, feel, etc.) with object
  Example: I saw him leave the house.

4. Verbs That Can Be Followed by Either

Some verbs can be followed by either gerunds or infinitives, sometimes with a slight change in meaning:

like, love, hate, prefer, start, begin, continue, forget, remember, try

Example of meaning change:

• I stopped smoking. (I quit the habit)
• I stopped to smoke. (I paused in order to have a cigarette)