Learn French: Pour améliorer ta capacité, tu dois bouger plus qu'avant. - To improve your capacity, you must move more than before.

When comparing verbs, the comparison words (plus que, moins que, autant que) go after the verb as a single unit. Here, we see bouger plus qu'avant (to move more than before). If you were comparing adjectives, plus and que would sandwich the adjective (like plus grand que).

Should I pronounce the 's' at the end of plus in this sentence?

Yes, you should pronounce the 's'. When plus has a positive meaning ("more") and modifies a verb, the 's' is always pronounced. This helps distinguish it from the negative ne... plus (no longer/not anymore), where the 's' is usually silent.

Why use Pour améliorer (the infinitive) instead of Pour que...?

When the person doing the action is the same in both parts of the sentence (it's "you" improving and "you" moving), we simply use pour + infinitive to mean "in order to". We only use pour que (which requires the subjunctive) when there are two different subjects.

Could I use devant instead of avant to mean "before"?

No, because avant and devant have different meanings. Avant is used for time (before in time, as it is here), while devant is strictly used for physical space (in front of).