Learn Spanish (Spain): El imperio romano era muy grande.

We use era (the imperfect tense of ser) because we are describing an ongoing characteristic in the past. It sets the scene and tells us what the empire was like over a long period of time, rather than focusing on a completed event with a specific end point.

What is the infinitive of era and why does it look so different?

The infinitive is ser (to be). It looks different because ser is one of only three completely irregular verbs in the imperfect tense (along with ir and ver). You just have to memorize its forms: era, eras, era, éramos, erais, eran.

Why isn't romano capitalized like 'Roman' is in English?

In Spanish, adjectives that describe nationalities, historical periods, or geographic origins are never capitalized unless they start a sentence. That is why 'Roman' is written with a lowercase 'r' as romano.

Imperio is a masculine noun. Why doesn't grande end in -o?

Adjectives that end in -e in their base form, like grande (big) or inteligente (smart), are gender-neutral. They use the exact same form for both masculine and feminine nouns. You only need to change them if the noun is plural, which would make it grandes.