What are the undrestood subject pronoun?

An 'understood subject pronoun' is called an implied pronoun, a pronoun that is not used in the sentence but is understood by the speaker and the listener (reader). Implied pronouns are acceptable when used in imperative sentences.

Examples:

'Stop!' (You stop.) 'Look! (You look.)

'Put the milk away. (You put the milk away.)

Implied pronouns can also be relative pronouns. Examples:

'This is the kind I like.' (This is the kind that I like.)

'This is the place I bought my car.' (This is the place where I bought my car.)

An incorrect use of implied pronoun: 'Hope this helps.'