quidam

quidam is qui plus the fixed particle -dam, and only the front half declines: quidam, quaedam, quoddam as an adjective and quiddam as a pronoun; genitive cuiusdam, dative cuidam, accusative quendam, quandam. That accusative is worth a look: the m of quem assimilates to the following d, which is why it is quendam and never quemdam.

Against aliquis it is the more definite of the two. aliquis is "somebody or other", identity unknown; quidam is "a particular somebody", identity known to the speaker but not being supplied. English "a certain" translates it exactly, and modern English's slightly arch use of "a certain Mr Smith" is the same move. In the plural it means "some, certain people", often with an edge, and it can also soften a bold metaphor, the way English says "a sort of".