Learn Spanish (Latin America): Sus pantalones están sucios.

Actually, it's the opposite! In Spanish, possessive adjectives match the thing being owned, not the owner. Because pantalones (pants) is plural, you must use the plural form sus. You would say sus pantalones whether they belong to one person (his/her pants) or multiple people (their pants).

So couldn't Sus pantalones also mean "His pants" or "Her pants"?

Yes, exactly! Su and sus have many meanings: his, her, its, your (formal), and their. Without extra context, this sentence could easily mean "His pants are dirty" or "Your pants are dirty." We just chose "Their" for this specific translation.

Why do we use están instead of son for "are"?

We use están (from the verb estar) because being dirty is a physical condition or state. You can wash the pants to change this state. If you used son (from ser), it would sound like being dirty is a permanent, built-in characteristic of the pants!

Why does the word for dirty (sucios) end with an "s"?