When you are learning german grammar, be careful with the past and present of the infamous `be` verb in german sein

sein (present)         sein(past)

ich               bin                     war

du                bist                    warst

er/es/sie    ist                       war

Wir              sind                   waren

ihr                seid                    wart

Sie                sind                    waren

Let us see the following sentences which often mislead those who are beginners in german. The answers to these sentences really depends on a clue which will say whether the sentence context is  present or past. Depending on that you have to use the sein verb.

Wo _____ du gestern?
Wo _____ du heute?
Ich _____ in Berlin.

_____ Sie schon mal in Berlin?
Ja, ich ___ schon mal in Berlin.