Use ab before words beginning with a vowel or h. Otherwise the form a can be used. It's a bit similar to English where we say an apple but a pear where an and a mean the same as each other, but using an before a word beginning with a vowel makes pronunciation easier.
I put "velis" and it didn't correct me. Apparently the algorithm used allows one wrong letter per word which is not helpful when learning as I then repeat the mistake later on and it gets imprinted on my mind. One of the moderators told me this.