Learn Finnish: Muistatko sinä heidät?

When a personal pronoun is the complete object of an action (the person having something done to them), it takes a special accusative form ending in -t. So the subject pronoun he (they) becomes the object heidät (them).

How is the word muistatko built?

It comes from the verb muistaa (to remember). First, we add the -t ending for "you" to get muistat (you remember). Then, we attach the question particle -ko to turn it into a yes/no question.

Since muistatko already means "do you remember", is the word sinä really necessary?

Not at all! Because the -t ending on muistatko already tells us "you" are doing the action, you could simply say Muistatko heidät?. Adding sinä just puts a little extra emphasis on "you," like asking "Do you remember them?".