Learn Spanish (Latin America): La exposición fue inaugurada al mediodía.

In the passive voice (using a form of ser + past participle), the past participle acts like an adjective. It must match the subject in gender and number. Since la exposición is feminine and singular, the participle changes from inaugurado to inaugurada.

Why use fue instead of era for "was"?

We use the preterite fue because the opening was a single, completed event that happened at a specific point in time (al mediodía). If you were describing a repeated past action or setting a background scene, you would use the imperfect era.

The lesson mentions using por (by) in the passive voice, but it's missing here. Why?

You only use por when you want to specify exactly who did the action. Here, the person who inaugurated the exhibition isn't mentioned because it's unknown or simply not the focus of the sentence. The sentence is still fully passive without it!

Why is it al mediodía instead of a el mediodía?

Whenever the preposition a (at/to) is followed directly by the masculine article el (the), they are forced to combine into the single word al. This is a mandatory contraction in Spanish.