Especially between two vowels, the Spanish b/v sound is often pronounced as a voiced bilabial fricative, which is a sound that doesn't exist in most pronunciations of English: https://en.wikipedia.org/wiki/Voiced_bilabial_fricative This can sound to English speakers like either a "b" or a "v" depending on context and expectations of the listener. And the pronunciation itself can vary by region or speaker as to how close it is to an actual English "b" or "v" sound. As far as Spanish speakers who actually make a conscious *distinction* between "b" and "v", they are often TV presenters and other people with media training, possibly influenced by English or other languages or maybe trying to speak that way for the sake of clarity. But the distinction is generally not made naturally among Spanish speakers.

L'expression me semble incomplète. Où l'avez-vous vue ? Sinon la locution "pour le ou la" s'emploie parfois devant un nom qui s'applique aux personnes et dont les formes masculines et féminines sont identiques afin de rendre le langage plus inclusif : "pour le ou la [biologiste, chimiste, secrétaire, locataire]"

Yes, "la clé / clef" is used metaphorically in French similarly to how it is used in English: something essential to finding a solution or making progress. * D’Artagnan est la clé de l’affaire des ferrets de la Reine. * L’informatique est la clé des industries de haute technologie. * Cette formation sera la clé de votre succès. See definition number 3 here: https://fr.wiktionary.org/wiki/cl%C3%A9

Que voulez-vous savoir exactement ? Le verbe "épuiser" vient du nom "puits" (un puits est un trou fait dans le sol pour tirer de l'eau, du pétrole, etc.).

The preposition “in” takes the accusative (den Supermarkt) when referring to a destination or movement “into” a structure or location. It takes the dative when referring to movement within a location where the person already is (im Meer schwimmen).

sich (Dativ) etwas vornehmen: etwas planen oder beschließen Beispiel 1: Nimm dir nicht zu viel vor! Beispiel 2: Für das neue Jahr hat er sich vorgenommen, mit dem Rauchen aufzuhören.

Wow, this is a really good question. Even as a native speaker I can’t think of a rule why we would say it one way or another. I just go with what “sounds” best.

The first one is correct because you need to put the verb in final position in a subordinate clause. Examples: „Weißt du, wie man diese 10 eingedeutschten Wörter schreibt?” https://www.welt.de/kmpkt/article183957172/Fuer-Deutschkenner-Weisst-du-wie-man-diese-10-eingedeutschten-Woerter-schreibt.html „Weißt du, wo die Tiere wohnen?” https://www.wohllebens-waldakademie.de/weisst-du-wo-die-tiere-wohnen-m54114