Use of "a" in this sentence...

Question

Why is there "a" before "nevar" in the following sentence:

Las nubes cubren el cielo y empieza a nevar.

Answer 1

empezar a (-ar, -er, -ir infinitive) is just a pattern for "to begin to ....."

The dictionary here even has a special notation for its usage:

empezar a hacer algo -> to begin or start to do something

Answer 2

Centain verbs in Spanish are followed by prepositions. See the list in the link below that require an "a" after them.

verbs followed by preposition

Answer 3

This link may help.