"Which horse is black?"
Translation:Quel cheval est noir ?
"Which" in English serves both as an adjective and a pronoun.
As with other adjectives, when "which" is an adjective, it must be paired with a noun, as in this example of "which horse is black?" In French, the adjective form of "which" is quel/quelle/quels/quelles.
As a pronoun, "which" is not paired with a noun. For example, if we were already talking about some horses, you could ask, "which is black?" or "which one is black?" In French, the pronoun form of "which" is lequel/lequelle/lesquels/lesquelle. Thus, "which is black?" would translate to "lequel est noir?"
Firstly, all nouns need modifiers, so "cheval" can't just hang out without one. Secondly, this is close to saying "Which is a black horse?", but you'd need "lequel" for that. "Lequel est un cheval noir?" Thirdly, "which is a black horse?" isn't the sentence you need to translate. http://french.about.com/od/mistakes/a/what.htm
"Which" when used as an adjective with a noun that it is describing is "quel", "quelle", "quels" or "quelles" depending on whether it is singular masculine or feminine, or plural masculine or feminine.
Now when "which" is used as a pronoun, sometimes it is "which one" or "which ones", and at times if you are talking about which person that would be the same as "who", but without context it could as easily be which thing and then "who" would not be applicable. The French pronouns for these would be "lequel", "laquelle", "lesquels" or "lesquelles" depending on whether it is singular masculine or feminine, or plural masculine or feminine. The pronouns are used instead of the nouns. http://french.about.com/od/grammar/a/adjectives_interrogative.htm http://french.about.com/od/grammar/a/lequel.htm