"We drank tea."
Translation:हमने चाय पी।
I think that in past tense the verb conjugates according to the object. Apparently चाय is feminine. So पी.
Duolingonaut is right. Transitive verbs in the past tense take the gender and number of their direct object.
पी is the feminine form, पिया is the singular masculine form and पिये is the plural masculine form. Since चाय is feminine, we need to use पी.