Translating "What is your name?" word by word: "Quoi est ton nom ?"
Use the fixed frame: Comment tu t'appelles ? / Comment vous appelez-vous ?
French asks "how do you call yourself", not "what is your name".
Mixing pourquoi (why) and parce que (because).
Pourquoi asks the question; parce que starts the answer.
They form a pair: Pourquoi… ? — Parce que…
Forgetting the space before ? in French typography.
French writes a space before ?, !, :, ; — Tu viens ?
It is the standard French typographic convention.
Translating 70, 80, 90 word for word from English.
Memorize the blocks: soixante-dix, quatre-vingts, quatre-vingt-dix.
France keeps the historic base-20 forms; "septante" works in Belgium/Switzerland only.
Writing "vingt-un" or "trente-un".
Use et for 21, 31, 41, 51, 61, 71: vingt et un, soixante et onze.
The et appears only in those "…and one" combinations.
Pronouncing the final consonants of cinq, six, huit, dix before a consonant.
Before a consonant they often drop: six livres "si livres", dix personnes "di personnes".
Number pronunciation changes with what follows; listening practice fixes this fast.