All Spanish adjectives that end in -e or -a in their masculine form are invariable

Question

This is a quote from kwiziq that is supposed to be explanatory, but it does not suggest a rule to know which adjectives have this form of ending. How are we to know which adjectives have this irregularity? Why can they not just follow the regular formula?

Answer

Hola Troyeb B
The intention of this lesson is to explain that there are adjectives that are invariable, i.e the masculine and the feminine form are the same. That is the case of adjectives that end in -e and -a. So if the masculine form (which is always taught as the basic form of the adjective) is -e or -a, you don't need to change the ending to apply that adjective to something that is feminine: 
el niño inteligente, el coche grande, el árbol verde...
la niña inteligente, la casa grande, la mesa verde...
el hombre idealista, el grupo comunista...
la mujer idealista, la chica comunista...
So, you see how with both masculine and feminine nouns, the adjectives keep their basic forms in -e or -a. 
Saludos
Inma