不能大声说话英语怎么说怎么写

“不能大声说话”的英文表达是 "speak softly" 或 "speak in a low voice".

"Speak softly" 意为说话声音较小，"speak in a low voice" 意为说话声音较低。

例句：

Please speak softly so you don't wake the baby. (请轻声说话，以免唤醒婴儿)
He always speaks in a low voice, so it's hard to hear him in a noisy room. (他总是说话声音低，所以在嘈杂的房间里很难听到他)

这个表达来源于英语中对语音音量的描述，并且在日常用语中很常见。