Learn Spanish (Latin America): El señor está triste por el accidente.

In this sentence, por indicates the cause or reason for his sadness. You use por when "because of" is followed by a noun (like el accidente). You would use porque (because) if it were followed by a full sentence with a verb, like "because he saw the accident" (porque vio el accidente).

Why not use para instead of por?

Por looks backward at the cause or reason (the accident caused the sadness). Para looks forward to a destination, goal, or purpose, which does not make sense for explaining why someone is sad.

Why is it está triste and not es triste?

We use está (from the verb estar) because being sad is a temporary mood or feeling here. If you said el señor es triste (using the verb ser), it would mean he is generally a sad or depressing person by nature.

Can señor just mean "man"? I thought it meant "Mr." or "sir".

Yes! While señor is used as a title like "Mr." (el señor Gómez) or a polite way to say "sir", in everyday Spanish it is also commonly used to simply refer to "the man" or "the gentleman," especially when talking about an adult.