event, incident / matter, affair / scandal

the lowdown, the inside story, inside information / the inner curtain of two curtains hung one within the other at a military camp

Same as above.

he / distal demonstrative, something far off removed from both speaker and listener: that / boyfriend

distal demonstrative, something far off removed from both speaker and listener: that, yon / particularly, male personal third person pronoun: he / by extension from he: boyfriend

he; boyfriend

to know, be aware of / to learn, acquire, / to be acquainted with, to know (someone) / (colloquial, in the negative) to care in the least about, to be concerned with,

stem or continuative form of 知る (shiru) [godan]

Alternative spelling of 物知り: a person with broad knowledge, a knowledgeable person

(literally) know the other side and yourself, a hundred battles will not be perilous