"My son likes the crib."
Translation:A mi hijo le gusta la cuna.
We don't have an equivalent in English which is why non native spanish speakers struggle (for awhile) with this concept. Perhaps it would help if I told you "a mi hijo" is what is optional in Spanish and that it is only added to clarify who is pleased. Le is not optional so think of the sentence like this "la cuna le gusta" and unless we know who le is then we add a ella, a usted, or whatever. In this case we are dealing with an indirect object pronoun. Did you notice the tips in the "object pronouns" module at the top of the screen once you start a lesson. It is quite concise and helpful. I don't know if I have given you the exact explanation you were seeking but I hope it helps.
It has been explained how 'a mi hijo' clarifies 'le gusta la cuna'.
However how fluid is the sentence structure? So, does the 'a mi hijo' have to be in the front; or is it sound as well to say le gusta la cuna a mi hijo; or la cuna le gusta a mi hijo?
Both sound a bit odd to my foreign ear, but then again - so does the correct sentence :-)
Think of "gustar" as the English verb "to please", because that's how you express "like" in Spanish. So "A mi hijo le gusta la cuna" is literally equivalent to "The crib pleases my son." Obviously, we don't speak like that in conversational English, but it's helpful to see how the verb "gustar" works. Here's another example: "My daughter loves basketball." = "A mi hija le encanta el baloncesto."