filmabas
-you were filming
The word filmabas is the imperfect form of filmar in the second person singular. There are other translations for this conjugation.
See the full filmar conjugation

filmar

A lo mejor no te diste cuenta, pero debiste pisarlo mientras filmabas.
You probably didn't realize but you must've done it as you were filming.
¿Por qué nos filmabas?
Why did you film us?
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