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Fourthly is a pointing word for lists. You have already given three points; this next idea is number four: “Fourthly, we need to save water.” It does that one job. You do not use it for a race or a place in a queue — there you say fourth (“she finished fourth”). It sounds a little stiff and old-fashioned, the kind of word you meet in a formal talk or a careful written argument. Many people now just say “fourth” or “number four” in the same slot, and that sounds more natural. Though actually, if you do use the -ly set (firstly, secondly, thirdly, fourthly), try to keep the set tidy; mixing “first” with “secondly” is common in real speech, but some careful writers dislike it.
第四點是…（列點時用）
組成fourth (number four in a series) + -ly (turns it into an adverb for listing). From fourth + -ly.
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。
fourthlyadv/ˈfɔːɹθ.li/
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In the fourth place; fourth in a row.
- Fourthly, I have made an effort to call the attention of the reader to the pertinent literature.
- Fourthly, when all equilibria are unstable and complex endogenous fluctuations arise, the LAW policy reduces the amplitude of fluctuations but at the expense of larger average deviations from the targets.
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。