"She assumed that he was with a different woman."
Translation:Ea a presupus că el este cu altă femeie.
What is your native languge? If it is not English, then check this link: https://webapps.towson.edu/ows/tenseconsistency.htm
In English, you must have use the past simple if the verb in the main sentence is at past simple, even if you refer to a present action.