Грамматика русского языка. Часть 8. Глагол. Глава 16. Разрешение, запрет, необходимость и т. д. Russian Grammar. Part 8. The Verb. Unit 16. Possibility, etc.

The infinitive is used in phrases to mean it is possible, it is impossible, it is necessary and it is time to.

(A) The key words expressing possibility and permission, impossibility and prohibition, necessity are: