how do you say......

[hola]

how do you say "and when you're done, im going to tell you why i ask/asked"

cuando terminaste, voy a decirte porque pregunto/pregunte

[Rusty]

how do you say "and when you're done, im going to tell you why i ask/asked"

cuando terminaste, voy a decirte porque pregunto/pregunte

It's hard to translate the final verb. We don't know the context. Here are two possible translations (with some invented context so you can determine if either is the right option):
Y cuando termines, voy a decirte por qué pedí que lo hicieras.
And when you're done, I'm going to tell you why I asked you to do it.

Y cuando termines, voy a decirte por qué hice la pregunta.
And when you're done, I'm going to tell you why I asked the question.

[Tomisimo]

Yes, the verb "to ask" trips up lots of new students :)

[CrOtALiTo]

how do you say "and when you're done, im going to tell you why i ask/asked"

cuando terminaste, voy a decirte porque pregunto/pregunte

I'll try of help you, I don't understand so well the context that you have putted to the ended the phrase When you are done? I thought that the word done could does not go here in this context but let see if I can translate this.

Cuando termines, voy a decirte lo que te pregunte.


It's not correcting good translate, but I hope can help you again.

have a good day

[Tomisimo]

im going to tell you why i ask/asked

Like Rusty said, this would be one of the following:

te voy a decir por qué te pregunté.
te voy a decir por qué te lo pedí.

[sosia]

The normal is (like Rusty says)
"y cuando (lo) termines, te diré porqué te lo mandé (hacer)"
"Y cuando termines, voy a decirte por qué pedí que lo hicieras"
you can use "terminar/acabar/finalizar/concluir"
and "hacer/mandar hacer/ordenar/pedir"

literal
"and when you're done, im going to tell you why i ask/asked"
"y cuando lo acabes, voy a decirte porqué lo pregunté"

saludos :D