• after verbs and expressions such as ask, learn, explain, decide, find out, want, want to know etc when they are followed by a question word. The maths teacher explained how to solve the problem.
Note: why is followed by subject + verb, NOT an infinitive. I wonder why she didn’t tell us.
• in the expressions to tell you the truth, to be honest, to sum up, to begin with etc. To tell you the truth I didn’t expect that you would come.
Note: If two to-infinitives are linked by and or or the to of the second infinitive can be omitted. I would like to go and see what is happening for myself.
The infinitive without to is used:
• after modal verbs. Beth can speak German and Italian.
• after the verbs let, make, see, hear, and feel. They made him fill out a lot of forms. BUT: we use the to-infinitive after be made, be heard, be seen etc (passive form). He was made to fill out a lot of forms.
Note: When see, hear and watch are followed by an –ing form there is no change in the passive. He saw me talking to Anna. I was seen talking to Anna.
• after had better and would rather. We had better take the train because the train because the traffic is very bad at the moment.
• help can be followed by either the to-infinitive or the infinitive without to. She helped me (to) choose the carpet for the living room.