Learn Spanish (Latin America): El soldado fue visto por la policía.

This sentence uses the analytic passive voice. To express that an action was done to someone, we use ser (here in the preterite, fue) plus a past participle. If we used estuvo, it would describe a physical location or a temporary state, rather than the action of being seen.

La policía is feminine. Why does the sentence use visto instead of vista?

In the passive voice, the past participle acts like an adjective that must agree with the subject receiving the action, not the person doing it. The subject is el soldado, which is masculine singular, so the participle must be visto.

Why do we use por to mean 'by' here? Could we use para?

In passive sentences, the agent (the person or group performing the action) is always introduced by por. You cannot use para here because para generally indicates a destination, deadline, or recipient, whereas por points to the cause or agent of the action.

Is visto a regular past participle?