Infinitives are generally used with the marker to. But we use the infinitive without to in some cases.

After modal auxiliary verbs

After the modal auxiliary verbs will, shall, would, should, can, could, may, might and must, we use the infinitive without to.

• I shall write to him. (NOT I shall to write...)
• We can manage it. (NOT We can to manage...)
• I will say what I like.
• We must get someone to paint the roof.
• Can you help us?

The infinitive without to can also be used after need and dare in some cases.

• You need not make such a fuss over it.
• How dare you call me a liar?

After do

After do, we use the infinitive without to.

• Do you think she is ready?
• I do admit that I was wrong.

After certain principal verbs like let, make, see etc.

After certain verbs like bid, watch, see, let, make and hear, infinitives are used without to.

• They made me wait. (NOT They made me to wait.)
• He bade me come. (NOT He bade me to come.)
• I saw her light the lamp. (NOT I saw her to light...)
• We heard her sing a song. (NOT We heard her to sing...)
• We watched them play.
• Let him go.

Help can also be used in this way.

• She helped the child (to) lift the box.

In passive versions of these structures the infinitive is used with to.

• She was heard to sing a song.
• He was helped to lift the box.
• She was made to pay back the money.

After rather, better and had better

After rather, better and had better, we use the infinitive without to.

• You had better consult a doctor.
• I would rather go alone.
• He would rather die than yield to pressure.

After and, or, except, but, than, as and like

When two infinitive structures are joined by and, or, except, but, than, as or like, the second is often without to.

• It is as easy to smile as frown.
• Do you want to have lunch now or wait till later?
• We had nothing to do except look at the cinema posters.

After why (not)

We can introduce questions and suggestions with why (not) + infinitive without to.

• Why stand up if you can sit down?
• Why pay more at other shops? We have the lowest prices.
• Why not make it up with him?

Note that no noun or pronoun should come after why (not).

• Why not arrange a party in his honour? (NOT Why not we arrange a party in his honour?)