The important thing to keep in mind when pronouncing the a is that its sound is always distinct, even when it's not stressed, unlike in English where unstressed vowels often receive a vague sound called the schwa. In the accompanying audio file, you can hear how a native speaker pronounces the a the same whether it's in adelante, where the a is stressed, and in gusta, where it is not.
Words or phrases spoken by native speakers in our brief audio lesson on pronouncing the a are adelante (go ahead), le gusta (he/she likes it) and otra vez (again).

