Learn Portuguese (Portugal): Eles lavam os copos e guardam-nos na gaveta. - They wash the glasses and put them away in the drawer.

The verb guardam ends in an "m", which makes a nasal sound. When attaching a direct object pronoun like os (them) to a verb that ends in a nasal sound, the pronoun changes to start with an "n" to make it easier to say. So, os becomes -nos.

Wait, doesn't nos mean "us"? How do I know it means "them" here?

You are right that nos normally means "us". However, because of the spelling rule from the previous answer, the pronoun os (them) turns into -nos after a nasal verb ending. You can tell it means "them" from the context: they are washing the glasses (os copos), not us!

Why use guardam for "put away"? Doesn't guardar mean "to keep" or "to save"?

Yes, guardar means to keep or save, but it is also the most common verb used for putting things away in their proper place, like putting dishes in a cupboard or clothes in a drawer.

Why is it na gaveta instead of em a gaveta?

In Portuguese, the preposition em (in or on) almost always shrinks and combines with the word that comes immediately after it. Here, it merges with the feminine word for "the" (a) to form na.