Hi Nessa, in Sindarin the verb is divided into two types, one is end up with -a, we call them A-verb. While the verb like mad- is another type called Basic verbs. These kind of verbs, when adding pronoun suffix, need to add an interim -i-. Thus, when you mean 'I eat', you should say 'medin'.
As about the second person suffix, you should be awared that in Sindarin, there is a polite(formal) second person suffix and an informal suffix. The polite one has single form '-(o)l' and the plural one '-(o)dh', the informal one only has single form '-(o)g'.
Here the (0) means that when A verbs conjugate, the ending -a will transfer into -o, like verb gosta- 'to fear'→ gostol 'you(formal) fear'(but in the mad- situation we don't need to because Basic verbs do not have vowel endings).
One other question is that you may doubt that when mad- 'to eat'→medil 'you(formal) eat', why the stem vowel a changes into e. This is quite complicated. In linguistics, this is a phenomenon called Vowel Harmony, where different vowels influence other vowels and some shifts may happen. Tolkein called it I-affection, because those vowel shifts in Sindarin often happen due to the interim -i-.
I-affection changes all vowels in the syllables except the last one, and has rules below:
a,o→e (example: talan 'tree'→telain 'trees')
e,i,y stay as the same
u→y
some o(to be precisely, the o from ancient ā) stay as the same
aw→ew
So here mad- actually becomes medil when you say 'you eat', or medig 'you eat' when in informal conditions.
