particle N5 essential casualpolitewritten
ambiguous — bare 'O' from vol B
A full written guide for ambiguous — bare 'O' from vol B is on the way.
This point is mapped in the grammar tree, but its examples, formation table and usage notes haven't been written yet. The fastest way to learn it now is to meet it in real sentences inside Jengo.
See this grammar in real sentences
Jengo shows each grammar point the way you actually meet it: inside real Japanese sentences, so it sticks instead of staying an abstract rule.
Study it in JengoSources Compiled with reference to A Dictionary of Basic Japanese Grammar. Editorial confidence: low.