Learn Russian: Толпа разошлась без конфликта.

The prefix раз- (or рас-) indicates movement outward or in multiple directions. When combined with the reflexive particle -ся (which becomes -сь after a vowel), it implies that the subjects are moving away from each other. When applied to a verb of motion, it turns the action of walking into scattering or dispersing.

Why is there an "о" in the prefix of разошлась instead of it just being разшлась?

The prefix раз- becomes разо- for easier pronunciation when attaching to certain verb roots, especially those that start with awkward consonant clusters. With the root verb идти (which becomes -йти when prefixed), it forms the infinitive разойтись. In the past tense, the root changes completely to -шл-, giving us the forms разошёлся, разошлась, and so on.

Since multiple people are scattering, why is the verb singular (разошлась) instead of plural (разошлись)?

Even though a crowd consists of many people, the subject noun толпа (crowd) is grammatically a feminine singular word. In Russian, the verb must strictly agree with the grammatical gender and number of the subject noun, so it takes the feminine singular past tense form.

What is the imperfective form of this verb, and how does the prefix spelling change?