partim

Partim is a fossil: an old accusative singular of pars that stopped declining and became an adverb, the same way that statim and olim froze. It cannot take an ending and never agrees with anything.

Its real habitat is the paired construction partim ... partim, "partly ... partly" or "some ... others", which is the neat Latin way of splitting a group without naming numbers. In that use it can govern a partitive genitive, partim militum, "some of the soldiers", a construction that betrays its origin as a noun and looks wrong to anyone treating it as a pure adverb. Compare the alii ... alii pattern, which does the same job with an adjective, and note that English "partly" only covers half of what partim does.