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'Thenceforth' means 'from that moment on, and continuing afterwards'. It marks a turning point in a story: something happened, and from then until the end of the account, things were different. 'In 1867 the land was sold to the city, and thenceforth the public could walk there freely.' It is built the same way as 'henceforth', which means from THIS moment on — the moment of speaking. That is the difference worth holding on to: henceforth points forward from now, thenceforth points forward from a past moment you have just described. Because of that, thenceforth almost always sits in narrative or history, while henceforth sits in announcements and rules. The register is high and old: wills, contracts, church history, nineteenth-century novels, formal chronicles. Even in careful modern writing it is unusual — most writers say 'from then on', 'after that', 'ever since'. Teach it as a word to recognise. If a student it in an essay it will not be wrong, but it will sound like they are wearing their grandfather's coat. You may also see 'thenceforward', which means the same thing and is, if anything, even heavier.
從那時起（古語/正式書面語）。日常說 from then on 就好；讀合約、史書、舊小說時會遇到
組成thence（從那裡/從那時）+ forth（向前）。So literally 'from that point, forward' — the old spatial word 'thence' stretched to cover time.
同家族的字
- henceforththe same build on 'hence' — from this moment on
- thencethe first half of the word
- forththe second half — forward, onward, as in 'set forth'
原始詞典資料1 筆記錄
以下是外部詞典記錄的用法，原文照列，我們還沒有寫成教學內容。上面的說明才是我們自己寫的；有些用法可能上面已經講過了。
thenceforthadv/ˌðɛnsˈfɔːθ/
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From that time on.
- ...to the end, that all such foes to the rights of British-America may be publicly known, and universally contemned as the enemies of American liberty; and thenceforth we respectively will break off all dealings with him or her.
- Furthermore: you must know that when the second iron is thrown overboard, it thenceforth becomes a dangling, sharp-edged terror, skittishly curvetting about both boat and whale, entangling the lines, or cutting them, and making a prodigious sensation in all directions.
資料來源：英文維基詞典 English Wiktionary（CC BY-SA 4.0）。 原文照列，未經改寫。