Learn Spanish (Spain): Las lavo rápido.

In Spanish, direct object pronouns like las (meaning "them") usually go right before the conjugated verb. Even though in English we say "I wash them", the Spanish structure is literally "Them I wash" (Las lavo).

How do we know to use las for "them" instead of los?

Las is a feminine plural pronoun. You use it here because whatever "them" refers to is a feminine plural noun (like las manos for hands, or las camisas for shirts). If you were washing masculine things, like los platos (the plates), you would use los instead.

Where is the word for "I" in this sentence?

The "I" is built directly into the verb! Lavo is the "I" form of the verb lavar (to wash). Spanish usually drops the subject pronoun yo (I) because the -o ending already makes it completely clear who is doing the action.

Since las is feminine and plural, shouldn't rápido be rápidas to match?

No, because rápido is describing how you wash (the action), not the objects themselves. Words that describe verbs are called adverbs, and adverbs never change their endings to match gender or number.