Learn Spanish (Latin America): La princesa quería el trono, por eso compró el veneno. - The princess wanted the throne, so she bought the poison.

In the imperfect, mental verbs like querer describe an ongoing state or feeling—she simply "wanted" the throne. If we used the preterite (quiso), the meaning would shift to describe a specific, completed action, usually translating to "she tried" or "she attempted" to get the throne.

Why is compró in the preterite while quería is in the imperfect?

This sentence uses a classic storytelling pattern. The imperfect (quería) sets the scene by describing the background motivation. The preterite (compró) describes the main event: a specific, completed action that moves the plot forward.

Does por eso always mean "so"?

In contexts like this, yes. Por eso literally means "for that," and it is the most natural way to say "so," "therefore," or "that's why" when connecting a cause to its resulting action.

Why is there no ella (she) before compró?

In Spanish, subject pronouns are often dropped when the subject is clear from context. Since la princesa is the subject of the first clause, it is obvious she is also the one buying the poison, making an extra ella unnecessary.