Hi
For 'chord' and 'phase' I don't think you can specifically speak of a base word; instead they're more like derivations I think.
Chord --> accord --> (Latin) Ad+cord --> 'cord' (heart) (here perhaps a suffix + base word)
Phase --> (Greek) Phasis (more like the original word but in another form)
However, in the case of 'assent':
Assent = probably derived from as+sent; to sent, which could be considered as the base.
Perhaps this is more the area of etymology, but it's interesting nonetheless.
Other words are sometimes more distinct and you can see more clearly that they consist of prefixes, infixes, and/or suffixes.
e.g. subsequently --> sub + sequent + ly --> sequence being the base word here.
Regards
Dokterjokkebrok