After many verbs English needs either a gerund (the -ing form) or an infinitive (to + verb). There is no simple logic, but the verbs fall into groups that you can learn.
These verbs are followed by -ing: enjoy, avoid, finish, suggest, mind, keep, practise, consider, admit, risk, miss.
The gerund is also used after any preposition: "She is good at drawing", "Thanks for helping", "before leaving".
These verbs take to + verb: want, decide, hope, plan, manage, refuse, promise, afford, agree, expect, learn, need.
Like, love, hate, prefer, begin, start, continue can be followed by either form: "I love swimming" and "I love to swim" mean almost the same.
| Verb | + gerund | + infinitive |
|---|---|---|
| stop | stop smoking (quit the habit) | stop to smoke (pause in order to smoke) |
| remember | remember locking the door (a memory) | remember to lock the door (not forget a task) |
| try | try adding salt (an experiment) | try to open the jar (make an effort) |
| forget | forget meeting him (no memory) | forget to call him (omit a task) |
To explain why you do something, use the infinitive: "I went to the shop to buy milk." Do not use "for buying" in this meaning.
The last example is a trap: here "to" is a preposition, so a gerund follows.