Learn Russian: Как только мама уснула, ребёнок заплакал. - As soon as mom fell asleep, the child started crying.

The phrase как только (as soon as) links two sequential events. Because Mom completely finished falling asleep before the baby reacted, we use the perfective past уснула. The baby's sudden reaction is also a single, completed event in the past, so we use the perfective заплакал.

How does the prefix in заплакал change its meaning?

The prefix за- is often attached to verbs to indicate the sudden start of an action, especially with sounds or emotions. While плакал just means 'was crying' (imperfective), заплакал means 'started crying' (perfective). This highlights the suddenness triggered by как только.

Could I use the verb спала instead of уснула here?

No, because спала means 'was sleeping', which is an ongoing imperfective process. The phrase как только requires a completed action to act as a trigger for the next event. уснула (fell asleep) provides that precise moment of completion.

Is the comma after уснула mandatory?

Yes! In Russian, you must always separate a subordinate clause (the part of the sentence introduced by как только) from the main clause with a comma. You cannot drop the comma like you sometimes do in English.