Learn Danish: Drengen vasker sit tøj. - The boy washes his clothes.

In Danish, if the subject of the sentence (the boy) is washing his own clothes, you must use a reflexive possessive like sit. If the sentence used hans, it would mean the boy is washing another boy's clothes!

How do I know whether to use "sin", "sit", or "sine"?

You look at the gender and number of the thing being owned. Here, the boy owns the tøj (clothes). Because tøj is a neuter noun (an et-word) and is singular, you must use the neuter form sit.

English uses the plural "clothes". Why aren't we using the plural "sine" for "tøj"?

In Danish, tøj is treated as a singular, uncountable noun, very similar to the English word "clothing". Because it acts as a singular noun, it takes the singular word sit instead of the plural sine.

I remember that "dreng" means boy. Why does it have "-en" at the end?

Adding -en to the end of an en-word makes it definite. So while dreng means "a boy", adding the suffix makes drengen mean "the boy".