"What happened to her toe?"
Translation:¿Qué le pasó a su dedo del pie?
is the "le" really necessary here? anf if so , can some one explain why? whats the rule?
The "le" means "to it" (the indirect object). Whenever you want to use a verb that requires an indirect object (I gave it to him, We spoke to/with her, he matters to me you) you need the indirect object pronouns in front of the conjugated verb/attached to the infinitive.
Without the IOP in front of the verb, it's like saying "something happened" (but not to anyone in particular), we spoke (to each other, not to him/her), and he is important in general, but not specifically to me. So the IOP in front of the verb is sort of like a tip-off to the listener that the verb in happening TO someone/something, not passively/generally occurring.
I just put dedo and was accepted. In the context of a conversation it would be understood just fine.
Yes. Dedo refers to finger and Dedo del pie is toe. You just have to memorize it.
Can't I say "Qué pasa con"? I know it's probably poor grammar but my native speaker friends never correct me when I say this.
Misuse of a preposition is probably not worth correcting. Friends are friends, so they know what you are trying to say, and they don't want to act as teachers. I have Hispanic friends who misuse English prepositions, and I am not about to correct them. If one says, "I have to park my car. I will see you in the corner by McDonalds." Do you think I am going to stand there and say "No, you will see me ON the corner, not in the corner!" Language is only a tool for communication. Life is short. Enjoy your friends.