Learn Spanish (Spain): La fiesta fue genial.

While estar is often used for temporary conditions or how someone feels, events like parties, concerts, or meetings always use ser when you describe how they went or where they happened.

Why use fue (completed past) instead of era (ongoing past)?

We use the preterite tense fue here because we are looking back at the party as one finished, completed event and summarizing it. If you used era, it would sound like you were setting the background scene for a story, such as: La fiesta era genial, pero... (The party was great, but...).

Doesn't fue also mean 'he/she/it went'?

Yes! The verbs ser (to be) and ir (to go) share the exact same forms in this past tense. You tell them apart by context. Here, the adjective genial follows the verb, so it acts as ser. If a destination followed it, like fue a Madrid, you would know it meant 'went' from ir.

La fiesta is a feminine word. Why doesn't the adjective change to geniala?

Adjectives that end in a consonant, like the l in genial, or an -e, like inteligente, have the same form for both masculine and feminine nouns. They only change when you make them plural, which would be geniales.