Breakdown of Si tu coupais le fil rouge, l'explosif exploserait.
tu
you (informal singular)
le
him, it (masculine direct object pronoun)
si
if
couper
to cut
Questions & Answers about Si tu coupais le fil rouge, l'explosif exploserait.
Why is coupais in the imperfect tense here instead of the present?
This sentence describes a hypothetical situation. In French, to express "if [X] happened, [Y] would happen," you use the formula si + imperfect tense + conditional tense. So the imperfect coupais sets up the hypothetical condition for the conditional exploserait.
Since we are learning about une fois que, how would this sentence change if I used it instead of si?
Using une fois que changes the meaning from a condition ("if") to a temporal sequence ("once"). Because this is a hypothetical scenario, you would need the conditional perfect to show the action is completed before the explosion: Une fois que tu aurais coupé le fil rouge, l'explosif exploserait (Once you had cut the red wire, the explosive would explode).