一面……，一面……: while… (two actions at once)
【高91】复句 / complex sentences (并列)
Tone colorsā tone 1á tone 2ǎ tone 3à tone 4a neutral
S + 一面 V1，一面 V2: one subject does two things at the same time, "while…": 他一面听老师讲课，一面认真记笔记. The subject goes before the first 一面, and each half is a verb or a verb phrase.
The pattern also links two measures taken together: 一面抢修，一面疏散 (repairing in an emergency and evacuating at once). It can even carry a reproach when the two actions do not go together: 他一面做作业，一面看电视，做事一点都不用心.
Register: somewhat more written than 一边……一边…… (hsk1-g47), which speech prefers. 一面 suits narrative, reports and talks: 大雁一面向南飞行，一面不断调整队形. Do not confuse it with the discourse pair 一方面……另一方面…… (hsk4-g09), which balances two arguments: here we have two simultaneous actions.
Examples
他一面听老师讲课，一面认真记笔记。
Tā yímiàn tīng lǎoshī jiǎngkè, yímiàn rènzhēn jì bǐjì.He listens to the teacher's lecture while taking careful notes.
他一面做作业，一面看电视，做事一点都不用心。
Tā yímiàn zuò zuòyè, yímiàn kàn diànshì, zuòshì yìdiǎn dōu bú yòngxīn.He does his homework with the TV on, and puts no real effort into anything he does.
救援人员一面清理道路，一面疏散居民。
Jiùyuán rényuán yímiàn qīnglǐ dàolù, yímiàn shūsàn jūmín.The rescue workers are clearing the road while evacuating the residents.
她一面照顾生病的母亲，一面坚持完成了自己的学业。
Tā yímiàn zhàogù shēngbìng de mǔqīn, yímiàn jiānchí wánchéng le zìjǐ de xuéyè.She looked after her sick mother while sticking with her studies to the end.
老师一面讲解，一面在黑板上画图。
Lǎoshī yímiàn jiǎngjiě, yímiàn zài hēibǎn shang huà tú.The teacher explains while drawing a diagram on the blackboard.
大雁一面向南飞行，一面不断调整队形。
Dàyàn yímiàn xiàng nán fēixíng, yímiàn búduàn tiáozhěng duìxíng.The geese fly south, constantly adjusting their formation as they go.
Common mistakes
✗ 一面他听课，一面我记笔记。 → ✓ 他一面听课，一面记笔记。 (one subject only, placed before the first 一面).
✗ 一面听课，另一面记笔记。 → ✓ 一面听课，一面记笔记。 (the second term is 一面, not 另一面; to contrast two arguments, say 一方面……，另一方面……).