Learn Hardcore Hebrew: אם את לא יודעת את הכתובת, תבדקי במפה בטלפון. - If you don’t know the address, check the map on the phone.

את at the beginning means you — specifically you addressed to one female.
את before הכתובת is the direct object marker. It has no English equivalent, but it is used before a definite direct object.

So in:

This is a very common source of confusion for learners.

Why is יודעת feminine?

Because the sentence is addressed to a female singular person.

Hebrew marks gender in many verb forms. Here:

Compare:

So the sentence is specifically talking to one female listener.

Why is תבדקי used here instead of an imperative form like בדקי?

אם את לא יודעת את הכתובת, תבדקי במפה בטלפון.