He conocido al banquero quien era hombre muy juicioso y prudente: I knew the banker who was a very wise and prudent man.
[Footnote 99: This is practically saying: after a comma, only the use of the comma is so arbitrary that we preferred to explain the rule fully.]
After ser and a noun or pronoun, translate "who" by quien, as—
Es él (Carlos) quien me lo ha dicho: It is he (Charles) who told it to me.
After a preposition translate "whom" by quien (pl., quienes)[100], and "which" by el cual (la cual, etc.).
[Footnote 100: "Que" is found but rarely.]
EXCEPTION—
After á, con, de, en, we can also translate "which" equally well by que, or el que, la que, los que, las que.[101]
[Footnote 101: "Que" or "El que" is found also after other prepositions but rarely.]
"Which" relating to a whole sentence is lo cual or lo que (naturally, because a whole sentence has no gender).