陆续 和 纷纷
one after another: 陆续 for a sequence over time, 纷纷 for many at once
Structure
主语 陆续 动词 （先后，一个接一个） · 主语 纷纷 动词 （很多人同时）
What it means
陆续 means one after another, with gaps in time; 纷纷 means many people doing the same thing at about the same moment.
How it is built
Subject + 陆续／纷纷 + verb: 亲戚陆续回来了 (relatives came back one after another); 网友纷纷留言 (people online rushed to comment). 陆续 can double: 陆陆续续.
When to use it
陆续 is about time and sequence; 纷纷 is about numbers, usually in a report of what has already happened. 纷纷 is also an adjective: 大雪纷纷, 议论纷纷.
Watch out
✗ 请大家纷纷入场。 ✓ 请大家陆续入场。 纷纷 describes what you see happening, so it cannot go in a request or a plan.
Examples
腊月底，在外地工作的亲戚陆续回来了。
Towards the end of the month, relatives working away from home came back one after another.
消息一传开，网友纷纷留言表示祝贺。
As soon as the news got out, people online rushed to post their congratulations.
会议九点开始，请大家陆续入场。
The meeting starts at nine; please start making your way in.
Learn this in Lesson 40 · HSK 5–6: 今天怎么过年 (The New Year today)
A dialogue, a reading, word cards with audio and a drill that uses it, in Learn Chinese with Bao. The first lessons of every level are free.
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