Pourquoi "m'assois" et non "m'assis" ?

Hi Michael ,

It’s because the only possibilities for, 

I sit down 

are:

Je m’assois/ je m’assieds 

Take a look at the following lesson for other forms - 

https://kwiziq.learnfrenchwithalexa.com/revision/grammar/conjugate-the-reflexive-s-asseoir-to-sit-down-in-le-present-present-tense

Hope this helps!