Intransitive verbs. An intransitive verb expresses action that is not directed toward an object. Therefore, an intransitive verb does not have a direct object.

The Red Sox persevered.

Their fans watched anxiously.

The verb persevered has no object (it makes no sense to ask, persevered what?), and the verb watched has an object that is implied but not expressed.

Some verbs that express action can be only transitive or only intransitive, but most can be used either way, with or without a direct object.

The verb opened is transitive here.

The door opened silently.

The verb opened is intransitive here.