I've been trying to figure out the differences between 一部, 一部分, and 部分 using GOO's thesaurus. It indicates that 部分 is used for one of several parts, while 一部 and 一部分 are for parts of a whole. And then it seems to indicate that while 一部 and 一部分 are the same, that 一部分 is just a bit more formal.
This makes sense to me, but then the examples with which are valid confuse me. The 4 examples are:
- 葉先の…が白い (all valid)
- 積み荷の…がくずれる (一部 and 一部分 are valid, but 部分 is invalid)
- …の人にしか伝わらない (一部 and 一部分 are valid, but 部分 is invalid)
- この文章で大切な…はどこですか (部分 is valid, 一部分 is questionable, and 一部 is invalid)
This definitely confuses me. The first feels like 部分 should be invalid, since the tip of a leaf can't be broken into multiple parts. The second, on the other hand, feels like it can be broken up into parts (a box/container of cargo), so it doesn't make sense why 部分 is invalid (it feels backwards). 3 explicitly implies the person is one of many others, so it again feels backwards. And the last is probably the most confusing of all of them. Sure, 部分 should work, since there are several parts to a sentence, but why is 一部 invalid, and more importantly, why is 一部分 semivalid? Shouldn't it always be as valid as 一部, apart from formality?