Rashi on Genesis 29:15/רש"י על בראשית כ״ט:ט״ו

1הכי אחי אתה This is a question: — IS IT BECAUSE THOU ART MY BROTHER THAT THOU SHOULDST SERVE ME FOR NOUGHT?

2ועבדתני is the same in sense as ותעבדני THAT THOU SHOULDST SERVE ME. So, too, in the case of every verb in the past tense, if one adds as prefix a ו it may change the verb to the future.