Learn Swedish: Jag skulle skrämma honom med sprattet. - I was going to scare him with the prank.

The aspectual till is used for sudden, brief, or involuntary actions, usually describing a quick reaction (like hoppa till for "jump/startle"). Here, the subject (jag) is deliberately and actively causing the scare, so till does not fit the verb. If you wanted to describe his sudden reaction to the prank, you would use a verb with till, such as han hoppade till (he was startled).

Why does skulle mean "was going to" here instead of "should"?

Skulle is the past tense of ska (will / is going to). It is frequently used to express the "future in the past," meaning an intention or plan you had at a previous time. While skulle can also act as the conditional "should," the context of planning a prank makes "was going to" the clear meaning here.

Could I use the s-verb form skrämmas instead of skrämma here?

No, because this sentence has a specific direct object (honom). The s-verb skrämmas is used when there is no direct object, to describe a general habit or characteristic of scaring people. For example, you would say han gillar att skrämmas (he likes to scare people), but han gillar att skrämma mig (he likes to scare me).

How do you use the root word spratt in a typical phrase like "to play a prank"?