Learn Spanish (Latin America): Él fue muy grosero y me ofendió.

We normally use the imperfect tense (era) to describe what someone is generally like. By using the preterite tense (fue) here, the speaker emphasizes that he was rude during one specific, completed event or interaction—he acted rude in that moment.

Is grosero related to the English word "gross"?

No, it is a false friend! Grosero means "rude", "impolite", or "vulgar". If you want to say something is gross or disgusting, you would use a word like asqueroso.

Why does me come before the verb in me ofendió?

Me is a direct object pronoun showing who received the action (who was offended). In Spanish, these pronouns must go directly before a conjugated verb. You cannot attach them to the end of a conjugated verb like ofendió.