Que: who, which, that
Every word has its English meaning underneath. English can drop “that” in the second and third examples; Spanish can never drop que.
After a preposition: quien and el que
A preposition is a small linking word such as con (with), en (in) or de (of, about). English often leaves it at the end of the sentence (“the friend I live with”). Spanish always moves it to the front, before the relative pronoun. After a preposition, use quien for people, or el que for people and things. El que agrees with the noun: el que, la que, los que, las que. El cual is a more formal equivalent.
Why: Quien is only for people. No article before médico: Spanish drops it before a job after ser.
Why: La que agrees with casa: feminine, singular. En la que can also be just en que.
Why: Por la que, “for which”, goes with razón and motivo (reason).
Why: Gente is a feminine singular noun in Spanish, so la que and es, where English says “are”.
Why: The verb is hablar de, “to talk about”; de + el que contracts to del que.
Lo que, cuyo, donde, en que
Que with the subjunctive: a thing you know, or one you are looking for
The verb after que is normally in the indicative, the ordinary form for facts. When the noun is something you have not found, or that may not exist, the verb switches to the subjunctive, the form Spanish uses for things not known to be real. The same sentence changes meaning with one vowel. More in the subjunctive.