how long has he been working? 结尾为啥省略for?
【答】句首单词应该大写。
for 用来说明持续的时间。
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在非正式文体里,常常省略 for,尤其是在问句 How long ...? 中。.........
How long have you been working (for)? 你等了多久了?
(摘自《牛津英语用法指南》第四版)