use of A mi
the sentence My son like the crib is translated A mi hijo le gusta la cuna. Why is the "A" used?
Gustar, as a verb, describes an object pleasing someone. When you say "me gusta x" you're saying that x pleases you. "Me" is an indirect object indicator. When you say "le gusta x," you're are saying that x pleases le, which refers to he/she/it and requires clarification. Given your example, "A mi hijo le gusta la cuna," you're saying "the crib pleases he/she/it, which is my son."