Full infinitive (to do)

English has three different infinitive forms. The infinitive we choose depends on the words that come before the infinitive or what the infinitive does in the sentence.

Reasons for the full infinitive

The full infinitive is used after some verbs

Examples: want, expect, would like, can't wait, hope, decide, agree, prefer, learn, arrange, agree, appear, arrange, ask, choose, manage, offer, plan, promise, refuse, allow, seem, try**, start**, begin**, continue**(there are many more!).

I want to go on holiday this year.

I expect to see you first thing tomorrow morning.

I'd like to buy an electric car.

I'm learning to speak Chinese.

**These verbs can also take the 'ing' form sometimes with a slight change in meaning.

It can be used after the verbs like and love when they suggest you do the action as a habit - what you usually do.

I like to have breakfast every morning while I read the news in my mobile phone.

We love to go away at weekends.

It is used to express purpose

You do something so that you can do another thing.

I went to the bank to take out some money.

My grandmother went to the shop to buy some tea.

Mary read the book to learn more about Winston Churchill.

So the full infinitive is always used after the expression in order to, which also expresses purpose.

We stopped going out to restaurants in order to save money.

The full infinitive is also used after many adjectives

Examples: pleased, surprised, happy, anxious, afraid, right, wrong, stupid, certain, careful, due, fit, able, lucky, difficult, pleased, important, happy, glad, nice etc.

My dog is very pleased to see you!

We were surprised to hear your news.

I'm so happy to meet you in person!

It is also used after some combinations of adjective + noun

I think that would be a silly thing to do.

It was a very clever move to make.

Chinese is a difficult language to study.

The full infinitive can be an adjective of a noun

Your comments have given him something to think about.

That was an important fact to learn.

It's a good film to see.

I didn't know what to think.

Full infinitives as the subject of a sentence

Many years ago in older English, it was quite common to use a full infinitive as the subject of a sentence.

To err is human, to forgive is divine.

In modern English, native speakers usually avoid this structure by using It...

It's normal to make mistakes.
It would be ridiculous to stop the project now.
It was very difficult to sell my house.

instead of the possibly more old-fashioned form,

To make mistakes is normal.
To stop the project now would be ridiculous.
To sell my house was very difficult.

More help on infinitives...

The bare infinitive...

Infinitive with ing (gerund)...