Relative pronouns advanced

Cuyo means 'whose' and behaves like a possessive adjective, so it matches the thing possessed and ignores the owner entirely. In el autor cuyas novelas leo, cuyas is feminine plural because novelas is, even though the author is a man. Getting that agreement right is the classic cuyo test.

When you are commenting on an entire situation rather than a named noun, que will not carry it. Llegó tarde, que me molestó is wrong: the genderless antecedent needs the neuter lo cual or lo que. The same applies when there is no noun in sight at all, as in no entiendo lo que dijiste.

Spanish never leaves a preposition at the end of a clause. It goes in front, and que then normally takes an article that agrees with the noun: la razón por la que, el amigo con el que, las cosas de las que. El cual does the same job in more formal Spanish.