María pide libros al profesor.
The 'al' seems to indicate 'to the' and yet the sentence would seem to call for a 'from the' as in 'from the professor'. So, I don't understand why 'al' is used in that place. Can you help me to understand. Thanks for you help.
2 Answers
I would say ... María le pide libros al profesor. - although I know it can be used without the le.
I'm not sure of it's proper grammatical term but the preposition 'a' in this case is just used to introduce the indirect object and it is untranslatable in English. (see No 5 below) Put with the 'el' it becomes 'al' but it isn't the usual prepositon 'a' meaning 'to'.
The preposition 'a' in Spanish has many uses and meanings including:
1) To indicate direction
Se acercó al perro. He got near to (approached) the dog.
Voy a casa. I'm going home.
Se cayó al río. She fell into the river.
2) To indicate position
María e sentó al sol. María sat in the sun.
Estabamos sentados a la mesa. We were sitting at the table.
3) To connect a verb with a following infinitive (not needed for all verbs but common after verbs indication the start of an action)
Empezó a nadar. She started to swim.
4) To indicate manner or method
With set expressions of 'a' + a noun - to indicate how something is done.
a pie = on foot
Voy al trabajo a pie. I go to work on foot.
a pie pelado (Chi) = barefoot
a mano = by hand
a dieta = on a diet
Estoy a dieta. I'm on a diet
a ciegas = blindly
Andan a ciegas. They are walking blindly.
a escondidas = secretly
Fumaba a escondidas de sus padres. She smoked behind her parents' backs.
5) The personal 'a'. (An untranslatable use. To introduce a direct object that is a person or treated like a person, eg a pet)
Conozco a Pablo. I know Peter.
Encontré a mi perro Rover. I found my dog Rover.
6) To introduce an indirect object
Le doy una bolsa a Jorge. I gave a bag to George.
**NB This 'a' does not mean 'to' here it's just introducing the indirect object. You can see this plainly in the following examples where we might expect to see 'para' / 'for', 'de' / from and 'en' / on instead of 'a' - but instead we just have the indirect object pronoun 'le' plus the 'a' + indirect object combination.
Le compro una bolsa a Jorge. I am buying a bag for George.
Le robo una bolsa a Jorge. I am taking a bag from George.
Le pongo la camisa a Paul. I am putting the shirt on Pablo.
7) In various time expressions
Cenamos a las seis. We are having dinner at six o clock.
Estamos a lunes. Today is Monday.
Hoy estamos a 20. Today is the twentieth.
A la mañana / A la noche In the morning / At night
I hope that helps somewhat 
I guess I should explain that I'm doing some stem-changing e:i verb drills on studyspanish.com and so far they have done this twice with the verb pedir and I don't know what to make of it. I am reading all comments and I hope to make some sense out of it. Thanks.
