Understanding when to use a Gerund (the -ing form) versus an Infinitive (the to + verb form) is one of the trickiest parts of English because it often depends entirely on the verb that came before it.
Think of it like a "verb marriage"—some verbs are only compatible with gerunds, others only with infinitives.
1. When to Use Gerunds (Verb + -ing)
Gerunds act like nouns. We use them in the following scenarios:
As the subject of a sentence: "Running is my favorite hobby."
After prepositions: "I am good at cooking."
After specific verbs:
Enjoy, Finish, Quit, Suggest, Mind, Avoid, Keep.
Example: "I enjoy swimming in the ocean." (Not: ~~I enjoy to swim~~)
2. When to Use Infinitives (To + Verb)
Infinitives often express purpose or a future intent. Use them:
After adjectives: "It is hard to learn Japanese."
To show purpose: "I went to the store to buy milk."
After specific verbs:
Want, Need, Hope, Decide, Plan, Promise, Agree.
Example: "I decide to leave tomorrow." (Not: ~~I decide leaving~~)
3. The "Identity Crisis" Verbs
Some verbs are flexible, but be careful—sometimes the meaning stays the same, and sometimes it changes completely.
Same Meaning
Verbs like Love, Hate, Like, and Start can take either with no real difference.
"I started to cry." = "I started crying."
Different Meaning
This is where it gets spicy. For a few verbs, the choice changes the timeline or the intent:

Verb	Followed by Gerund (-ing)	Followed by Infinitive (To)
Stop	To quit an action permanently. (I stopped smoking.)	To pause what you're doing to do something else. (I stopped to smoke.)
Forget	To lose a memory of the past. (I forgot meeting him.)	To fail to do a task. (I forgot to meet him.)
Try	To experiment with a new method. (Try adding salt.)	To make an effort at something difficult. (Try to lift this.)

Quick Cheat Sheet
If you aren't sure which one to use, ask yourself: Has the action happened yet?
Gerunds often feel like they are happening "now" or have already happened (real/vivid).
Infinitives often feel like they are "ahead" of you (potential/future).
Pro Tip: If you see a preposition (in, on, at, with, about, before, after), the very next verb must be a gerund.