Learn Spanish (Latin America): Yo clasifico todo lo que no usamos.

We use the indicative (usamos) because we are talking about a known, established reality. The speaker is referring to specific things that they already know they don't use in their current everyday life.

How would this sentence change if the items they won't use are currently unknown?

If you are talking about things you might not use in the future, the unknown nature of those items triggers the subjunctive. For example, if you are packing for a trip and say, "I will sort everything we (end up) not using," you would say, "Yo clasificaré todo lo que no usemos."

Why do we need the lo in todo lo que? Can't I just say todo que?

In Spanish, you can't put todo (everything) directly next to que (that) to mean "everything that." You need a neutral pronoun in the middle. Lo que means "that which" or "the things that," so todo lo que literally translates to "all that which" or "everything that."

Clasifico looks like "classify." Is it commonly used to mean "sort"?

Yes! While clasificar certainly means "to classify," it is also the standard, everyday verb used for "to sort" things into categories (like laundry, documents, or items you plan to donate). Another common verb you might hear for generally sorting or organizing a room is ordenar.