"Are you wearing fake nails?"
Translation:Tu portes de faux ongles ?
Shouldn't it be "portez-vous de faux ongles". Tu portes de faux ongles says You wear false nails. That is not a question.
"Tu portes" is less formal than "Portes-tu" but both can be questions.
Similarly (although not exactly the same), in English, you can ask either "You have the time?" Or "Do you have the time?" The first phrase is less formal, and can be phrased as a statement if you use different punctuation, but it is nevertheless a question.
Is this another instance where it means two different things with the adjective before or after the noun?
We would not use "faux" after "nails", because it is a fixed phrase, also used with a lot of other "fake" things: de la fausse monnaie (counterfeit money), de la fausse fourrure (faux-fur), du faux cuir (artificial leather), etc.
In spoken French, you can use "des faux ongles" but in proper French, "des" becomes "de" before an adjective.
And would it be true to say that "de" is always used instead of "de la, du, and des before any adjective? Or does it only apply to "des"?
Only in specific instances do "du, de la" and "des" change to "de", like negative sentences:
- je mange du pain --- je ne mange pas de pain
- je bois de l'eau --- je ne bois pas d'eau
- j'ai un chapeau --- je n'ai pas de chapeau
- je porte des chaussures --- je ne porte pas de chaussures.
The change from "des" to "de" before an adjective is only before a plural adjective:
- un faux ongle --- de faux ongles
- du bon pain
- de la belle salade