The infinitive should be used if it directly proceeds an adjective. Examples:
- I am too tired to swim.
- She was happy to help.
- We are too fast to be caught.
The gerund form should be used if it follows a preposition. Examples:
- I left the party without saying goodbye.
- We look forward to winning the game.
There are, as with anything in English, numerous exceptions that may not seem obvious. Certain verbs require the infinitive form whereas others seem more appropriately conjoined with the gerund form. For example:
- Would you mind getting me a towel? | (verb, gerund)
- I forgot to bring my phone. | (verb, infinitive)
- I regret sleeping with Josephine. | (verb. gerund)
- She cannot afford to go on vacation. | (verb, infinitive)
Often times, they are interchangable:
- He said he likes to smoke.
- He said he likes smoking.