忽A忽B
now A, now B; suddenly A, then suddenly B
Structure
忽 A 忽 B
What it means
Now A, now B: quick, unpredictable swings between two one-syllable opposites, usually as a complaint.
How it is built
忽 + A + 忽 + B (高/低, 冷/热, 明/暗, 快/慢, 大/小, 远/近): 这几天气温忽高忽低，很容易感冒 (The temperature has been up and down; it’s easy to catch a cold).
When to use it
A four-character frame (四字格) for unstable things. 忽冷忽热 is common about feelings: affectionate one day, cold the next (他对女朋友忽冷忽热).
Watch out
No two-syllable words: ✗忽高兴忽难过 ✓一会儿高兴，一会儿难过.
Examples
这几天气温忽高忽低，很容易感冒。
The temperature has been up and down these last few days; it is easy to catch a cold.
他对女朋友忽冷忽热，让她心里很不踏实。
He blows hot and cold with his girlfriend, which leaves her feeling very unsure.
山路上的灯光忽明忽暗，看得人心里发慌。
The lights on the mountain road kept flickering, which made us nervous.
Learn this in Lesson 223 · HSK 9+: 张爱玲：红玫瑰与白玫瑰 (Eileen Chang: red rose, white rose)
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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