Con quien, en la que: after a preposition

After a preposition: con quien, en la que

When a preposition (con, de, a, para) comes before the relative, a person takes quien, or quienes for more than one:

Notice where the preposition goes. English leaves it at the end (the woman I talk to); Spanish puts it in front of the relative.

For a thing, the everyday choice is el que, la que, los que or las que, agreeing with the noun:

After en, plain que also works for a thing: la casa en que vivo. A person can take the article form too: la mujer con la que hablo means the same as con quien. What Spanish avoids is bare que for a person after a preposition.

Commas: que or quien

A clause in commas adds an extra fact about someone already identified. Here que and quien are both correct for a person, and the difference is register:

• Mi padre, que vive en Madrid, viene mañana. - My father, who lives in Madrid, is coming tomorrow.
• Mi padre, quien vive en Madrid, viene mañana. - The same, a touch more formal.

The formal cousins el cual and la cual are on advanced Spanish relative pronouns.