Infinitive as subject or complement

Subject: To practise is important. / It’s important to practise.

An infinitive (or a clause beginning with an infinitive) can be be the subject of a sentence.

• To practise regularly is important.
• To wait for people who were late made him angry.

In modern English, this is unusual, and especially unusual in an informal style. We more often use a structure with it (see here).

• It’s important to practise regularly.
• It made him angry to wait for people who were late.

We can also use an -ing structure at the beginning of a sentence as the subject, instead of an infinitive (clause) (see here).

• Selling insurance is a boring job. (More natural than To sell insurance …)

Complement: Your task is to get across the river.

An infinitive (clause) can be used after be as a subject complement.

• Your task is to get across the river without being seen.
• My ambition was to retire at thirty.

Sentences like these can also be constructed with preparatory it (see here).

• It is your task to get across the river without being seen.
• It was my ambition to retire at thirty.

note

For infinitives or infinitive clauses as the objects of verbs, (see here).

For structures like He made it difficult to refuse, (see here).