Both pedir and preguntar mean ‘to ask’ but they are not interchangeable. Preguntar means ‘to ask a question’: Roberto le pregunta dónde está la oficina (‘Roberto asks her where the office is’). Pedir means ‘to ask for something,’ and is also used to mean ‘to order’ (in a restaurant): Roberto le pide información (‘Roberto asks her for information’). Roberto pide los tacos (‘Roberto orders tacos’).

Pedir is slightly irregular, and should be memorized.

PEDIR (to ask for, to order)