Hola R

I'm not sure if you mean when to use pedir in the present or past tense or when to use a specific tense in the subjunctive in the subordinate clause? Can you clarify that?

If what you mean is how to know when to say:

Te pido que me ayudes vs Te pedí que me ayudaras

this depends on whether you are making a statement in the present or in the past, "te pido que..." or "te pedí que..."; every time you use it in the present tense, the natural sequence of tenses is "present indicative (main) + que + present subjunctive"; every time you use it in the past tense, the natural sequence of tenses is "past tense indicative (main) + que + imperfect subjunctive";

This is what you also find in for example:

"Espero que seas feliz." vs "Esperaba que fueras feliz.", or "Quiero que te vayas." vs "Quería que se fuera."

Is this what you meant?

Saludos

Inma