[Rusty]

Originally Posted by cochinillo

Can you say "Que te lo/la pases bien"? Yes. 'Lo' is used in Spain. 'La' is used in Latin America. The reflexive pronoun is also necessary because the verb is pronominal - pasársela.
And in this instance you use pases instead of pasas because it is subjunctive, correct? Yes.
Another way to say it could be "Espero que tengas un buen cumpleaños" - again with the subjunctive. Would this turn into "Tenga un buen cumpleaños" - as a sort of command form? Or "Ten un buen cumpleaños." The second person would be used because you used 'tengas'. Both the second-person and third-person forms are used, of course. It depends on the audience.
If I remember correctly, "ten" is the informal (2nd person, tú) command form for 'tener'. Do you only use "tenga" for affirmative commands and "no tengas" for negative commands? 'Ten' is the second-person affirmative command and 'no tengas' is the negated form. 'Tenga' and 'no tenga' are the third-person equivalents.

The command form is also called the imperative.