Prefix used in formal or written contexts to refer to the present instance of a period, event, or thing. Common in compounds like 今世紀 (this century), 今シーズン (this season), 今国会 (the current Diet session).
今世紀最大の発見と言われている。
It is said to be the greatest discovery of this century.
today's
Prefix used in formal or written contexts to refer to today's instance of something. Common in compounds like 今朝 (this morning) and 今夕 (this evening), though those are often read with kun'yomi. The こん reading appears in more formal or technical compounds like 今暁 (this dawn) or 今般 (this time/recently).
今般の決定についてご報告申し上げます。
I would like to report on the recent decision.
今シーズンの目標は優勝です。
Our goal for this season is to win the championship.
今暁の気温は氷点下まで下がった。
The temperature dropped below freezing this dawn.
現 emphasizes the present ongoing state of something (現状, 現在) whereas 今 as a prefix marks the current specific instance of an event or period (今国会, 今世紀).
今日 is the independent everyday word for 'today', while 今 (こん) as a prefix appears only inside formal compounds referring to today's instance of something, like 今暁 or 今般.
旧 as a prefix means 'former, old' (旧校舎, 旧姓), the opposite of 今's sense of 'current, this' (今シーズン, 今国会).
The kanji 今 means 'now' or 'present'. The reading こん is the on'yomi (Sino-Japanese reading), used in many compounds to indicate the current or present instance of something.
this season; the current season
these days; nowadays; recently
tonight; this evening
this term; this period; current session
this fiscal year; this school year; this year
ancient and modern times; all ages; past and present
this summer; next summer; last summer
this century
this autumn; this fall
this spring; spring this year
this winter; next winter; last winter
past and present; ancient and modern
this life; this world
now; nowadays; the present time
the present day; the present age; nowadays
now; recently; this time
this season
this world; this life · this generation; present day
present-day people; people of this world
this work; this title · modern work not based on old styles
this issue; this number
one's present self