I don't understand the point of the lesson. When do you ad an a and when don't you?

Hola Bo

The lesson is saying that the general rule is to take the feminine form of the adjective (ending in -a, e.g rápida, mágica, clara, ...) and add -mente, but if the adjective is the same for masculine and feminine, you just take that form, e.g feliz, legal, elegante... and add -mente to it. 

rápidamente, mágicamente, claramente...

felizmente, legalmente, elegantemente... (these don't have a form for masculine and a form for feminine, they are the same, so "la mujer feliz, el hombre feliz" for example.

Hope this helps

Saludos

Inma

Hola Karen

I'm afraid not; it is a question of memorising those and using them a lot when you practise. 

Inma