Learn Italian: Nella preistoria, l'estate era spesso corta. - During prehistory, the summer was often short.

Era is the imperfetto (imperfect) tense of essere (to be). We use it here because we are describing a general, ongoing state in the past—how things "used to be" over a long period. If we used the passato prossimo (è stata), it would sound like a single, specific summer that started and ended, rather than a description of an entire era.

How do I know the gender of estate (summer) since it just uses l'?

You can look at the adjective! Estate is a feminine noun. Because it starts with a vowel, the article is shortened to l', which hides the gender. However, the adjective corta (short) at the end of the sentence ends in -a, confirming that estate is feminine singular.

Why is it Nella preistoria instead of just In preistoria?

In Italian, you usually need to include the definite article (the) when talking about specific historical eras, dates, or broad concepts. Nella is a combined preposition made by merging in + la.

How could I change this sentence to say "A long time ago, summer was often short" using fa?