#1
when do i use a suspended chord? and do all chords have to be suspended if one is?
#2
it's used to create tension. i like to use 'em as the second-to-last chord in chord progressions of like 6 or 8 or more chords. but whatever sounds cool to you is when you could use 'em.

and dont make make them allsus chords. it sounds off. unless it sounds good to you, like i said.
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#3
There are two sus chords, sus2s and sus4s. A sus2 is formed by the first, second and fifth(1-2-5) notes of a scale while a sus4 is a first, fourth and fifth(1-4-5). They can give a tense feeling and are usually followed with a major chord.