> 'Feynman moment'- "If you cannot explain something in simple terms, you don't understand it".

Unsure if you're attributing it to Feynman (although I do see the relationship in Feynman's thought process, but the quote is from Einstein:

> If you can't explain it simply, you don't understand it well enough.

... buuut really it's from Nicolas Boileau (1674):

> Ce que l'on conçoit bien s'énonce clairement. Et les mots pour le dire arrivent aisément.

Tx'd:

> What is well understood is told with clarity, and the words to say it come up easily.

As well as a few others that are right on point:

> Avant donc que d'écrire, apprenez à penser. Selon que notre idée est plus ou moins obscure, l'expression la suit, ou moins nette, ou plus pure.

Tx'd:

> Before even to think one should learn to write. Whether the idea is more or less obscure, expression follows through, 'ther less sharp or more pure.