Learn Spanish (Latin America): La actriz trabaja en el teatro.

The masculine form is el actor. While many Spanish words change from -o to -a to become feminine, professions ending in -or often have special feminine forms. Here, actor becomes actriz.

Why do we use en el teatro to mean "at the theater"?

The Spanish word en covers "in," "on," and "at" in English. When you are talking about being located at a place, you use en. If the actress were moving toward the theater, you would use a (to).

Why isn't "the theater" translated as lo teatro?

Teatro is a masculine noun, so it uses the standard masculine word for "the," which is el. A very common beginner mistake is using lo, but lo is only used for abstract concepts (like "the good thing"), never in front of standard nouns.

Which verb is trabaja from?

It comes from the regular verb trabajar (to work). Because we are talking about la actriz (she), we drop the -ar ending and add -a to make trabaja.