Learn French: Ils ont fait le sprint en dix secondes. - They did the sprint in ten seconds.

In French, en describes the amount of time it takes to completely finish an action. Because it took exactly ten seconds to finish the sprint, we use en. If we wanted to say when an action will begin in the future, we would use dans.

What would happen if we swapped en for dans in this exact sentence?

Using dans here would sound very strange because the verb ont fait is in the past tense. The word dans is used to say how much time is left before an action starts. If you changed the verb to the future tense to say Ils feront le sprint dans dix secondes, it would make sense and mean they will start the sprint ten seconds from now.

Could I use pendant instead of en in this sentence?

It would sound unnatural. The word pendant focuses on the duration of an ongoing activity, meaning 'for ten seconds'. The word en focuses on the time needed to achieve a specific result. Because finishing a sprint is a completed goal, en is the correct choice.

How do you pronounce the number dix in the phrase dix secondes?

The x at the end of dix is completely silent here because the next word, secondes, starts with a consonant sound. If the following word started with a vowel, it would make a 'z' sound, and if it were at the very end of a sentence, it would make an 's' sound.