Question

i don't know what an understood pronoun is

Answer 1

The only understood pronoun is the subject "you" in a command or request. Consider these examples:

Pass the butter, please.
Take out the trah.

In both sentences, we understand that "you" is the subject of the sentneces.

Answer 2

An "understood pronoun" is usually referred to when a sentence is in imperative (command) form.

In the following sentence, there is no WRITTEN SUBJECT:
Open your books to page 51.

The verb is "open" and the rest of the sentence is the direct object and its modifiers. In other words, the whole thing is the complete predicate -- the verb and everything that goes with it. But even though the subject is not stated, do you have any doubts about who is supposed to open their books?

Formally, "you" is the understood pronoun that serves as the unwritten subject of any command. This is the pronoun I think you're referring to, right?

Answer 3

An understood pronoun is when you talk to someone but leave out the word "you", because its obvious that you are talking to them. Example: "You pick that up" could be said as "Pick that up" In the second sentence the "you" is unserstood, so it doesnt need to be said.