Maybe I can help: The problem is that when you hear the sentence for people learning German it can be either "was ist du" or "was isst du"
You can forget about the first one "was ist du?" because the sentence does not make any sense and therefore no one will use it.
That leaves the the second version: "Was isst du?"; it comes from the German verb "essen"="to eat" and means "what do you eat?" (using all personal pronouns subject case you get: was esse ich , was isst du, was isst er/sie/es, was essen wir, was esst ihr, was essen sie).
Looking at the sentences just presented you can see that the only sentences that might present a problem are 2nd person singular and third person singular. But knowing how they are spelled and know what they can mean they should provide no problem.
Hope that helps you. :-) Enjoy