Learn Russian: По мере того как проходит время, старый трос должен быть заменён. - As time passes, the old cable must be replaced.

This is the passive infinitive construction you are currently studying. The modal word должен (must) agrees with the masculine singular subject старый трос. It is followed by the infinitive быть (to be) and the short past passive participle заменён (replaced), which also agrees with the masculine singular subject. Because it is formed from a perfective verb, it emphasizes the completed result of the action.

По мере того как seems like a lot of words just to say 'As'. How exactly does this phrase break down?

This is a common compound conjunction at the B2 level. It literally translates to 'by the measure of that how', but it smoothly means 'as', 'as long as', or 'in proportion as'. It is used specifically to show that two processes are happening simultaneously or that one changes progressively in response to the other.

Why is the word order проходит время (passes time) instead of время проходит?

In Russian, putting the verb before the subject often highlights the action or the existence of the subject rather than the subject itself. Here, the focus is on the passing of time as a background process, making проходит время sound much more natural in this dependent clause than the standard English subject-verb order.

Why use the short form заменён instead of the long form заменённый?