"I was at his house last month."
Translation:Estuve en su casa el mes pasado.
When ¨pasado¨ is used (and comes after the time noun like month, year, etc), think of ¨pasado¨ as meaning ¨that has passed.¨ So ¨el mes pasado¨ is ¨the month that has passed¨ in English. It could not be just ¨month that has passed.¨ I hope this helps with remembering! (and I realize that you posted 2 months ago, so if you don´t need this anymore, congrats!)
Estaba and estuve are both accepted here. Correct me if I'm wrong, but;
Would "estaba" imply that the speaker was there for the whole month, i.e. spent the whole month living there, while "estuve" would imply that he was there once or some miscellaneous number of times during the last month?