When I teach you on my fingertips
a "if only I could be my never tiring fingers".
I know you by heart
a "why are you not what he thinks you are?" Just some surface
Come and learn1 me by heart
a "if only you could be nothing but your fingers, may2 your enamel fade away".
1. could also be meant as "teach me until I know by heart"2. this is weird French indeed, and I'm not sure that's the intendend meaning. It could either be "ton émail qui s'efface" with "qui" misspelled as "que" (your enamel that fades away) or "que ton émail s'efface" with "que" displaced yoda-style in the middle of the sentence (may your enamel fade away)