After some verbs we use -ing (gerund); after others we use 'to + infinitive'. Some verbs allow both — sometimes with a change in meaning.
Verbs followed by -ing (gerund)
enjoy, mind, finish, suggest, avoid, keep, waste (time/money), stop (an activity), regret, deny.
- You shouldn't waste time on this.
- He hadn't better waste his time.
- She doesn't have to take her medicine. ← infinitive after 'have to'
Verbs followed by 'to + infinitive'
want, need, hope, decide, plan, promise, try, agree, refuse, learn, able to, ready to.
- Have you been able to work?
- Steve must not need to stop wasting his precious time.
Stop + ing vs stop + to
'Stop smoking' = quit smoking. 'Stop to smoke' = pause to have a smoke. Same verb, different meaning.
Need / order to / so as to + infinitive
'In order to', 'so as to', 'so that' all express purpose — followed by an infinitive (or a clause for 'so that').