Learn Russian: Как только кот испугался, он спрятался.

The phrase как только means "as soon as" and highlights that one action happens immediately after another. Because the cat fully completed the action of getting scared before it hid, both verbs are in the perfective aspect to show they are sequential, finished events.

What is the purpose of the -ся ending on both verbs in this sentence?

The suffix -ся makes the verbs reflexive, showing that the action affects the subject itself rather than an outside object. Without it, you would need a direct object: for example, you would be hiding a toy instead of hiding yourself (спрятался), or scaring a mouse instead of getting scared (испугался).

Why is the pronoun он used for the cat instead of a neutral pronoun for "it"?

In Russian, personal pronouns always match the grammatical gender of the noun they refer to. Because кот is a masculine noun, it must be replaced by он, even though English typically uses "it" for animals.

Is the comma after испугался mandatory?

Yes. In Russian grammar, a subordinate clause (the dependent part of the sentence starting with как только) must always be separated from the main clause by a comma. This comma marks exactly where the main action begins.