Learn Russian: Как только гости приехали, собака спряталась. - As soon as the guests arrived, the dog hid.

The phrase как только (as soon as) is used here to show that one completed event immediately triggered another. Because both actions are finished, one-time events—the guests arrived, and then the dog hid—we use the perfective aspect for both.

Why is the verb спряталась reflexive (ending in -сь)?

In Russian, the base verb спрятать means "to hide something else" (like hiding a bone). To say that the dog hid itself, you need the reflexive suffix. Since the past tense feminine form (спрятала) ends in a vowel, the standard -ся suffix shortens to -сь.

Could I use the imperfective verb ехали instead of приехали here?

Not in this context. The imperfective ехали means the guests "were traveling" or "were on their way". To trigger the dog hiding "as soon as" they did something, their arrival must be a completed action. The prefix при- adds the meaning of arrival and makes the verb perfective.

Is the comma after приехали strictly necessary?

Yes, it is mandatory! Unlike English, where you can sometimes skip commas, Russian punctuation rules strictly require you to separate a subordinate clause (the part starting with как только) from the main clause with a comma.