早知今日何必當初

if I (you, she, he...) had known it would come to this, I (you, she, he...) would not have acted thus (idiom) / to regret vainly one's past behavior

early / morning / Good morning! / long ago / prematurely

foresight / foreknowledge / if one had known earlier,...

to know / to be aware

now / the present time / current / contemporary / this (day, year etc)

today

abbr. for 日本[Ri4 ben3], Japan

sun / day / date, day of the month

surname He

what / how / why / which / carry

there is no need / why should

certainly / must / will / necessarily

to be / to act as / manage / withstand / when / during / ought / should / match equally / equal / same / obstruct / just at (a time or place) / on the spot / right / just at

at or in the very same... / suitable / adequate / fitting / proper / to replace / to regard as / to think / to pawn / (coll.) to fail (a student)

at that time / originally

at first / (at the) beginning / first / junior / basic