Standard French numbers include one of the more famous grammatical quirks among European languages: a genuine historical remnant of counting in twenties rather than tens for some numbers.
Quatre-vingts ("eighty") literally means "four twenties" (quatre = four, vingts = twenties) — a real base-twenty (vigesimal) counting remnant, not a simplification. Soixante-quinze ("seventy-five") similarly means "sixty-fifteen" (soixante = sixty, quinze = fifteen), continuing the same non-decimal logic for numbers in that range.
Belgian and Swiss French commonly use septante (seventy) and nonante (ninety) instead of the standard French soixante-dix and quatre-vingt-dix — genuine, standard regional alternatives, not errors, reflecting a more straightforwardly decimal counting system in those varieties.
Numbers up to sixty-nine follow a fairly standard additive pattern similar to English — the base-twenty quirks are specifically concentrated in the seventy-to-ninety-nine range in standard French.
Because standard French retains a genuine historical base-twenty (vigesimal) counting remnant for that range — quatre-vingts literally means "four twenties," not a simplification or error.
Standard (especially French, France) usage does, but Belgian and Swiss French commonly use septante (seventy) and nonante (ninety) instead — both are genuine, standard regional forms.
No — numbers up to sixty-nine follow a fairly standard additive pattern similar to English; the base-twenty quirks are concentrated specifically in the seventy-to-ninety-nine range.
Yes, as core vocabulary content, matching the approach used in Tarc's existing courses.
See also: French Vocabulary · French Grammar · Learn French