1. It is a week since he came here last time.
2.How long hasn't he come here? 第三人称要用has ,要用现在完成时。
3. How many days hasn't he come here ?这种说法是对的，for在疑问句时就省略了。
1. 同义句：He has disappeared for a week
2. It is... since只是说since后面的从句不能用否定
I haven't seen him since 3 years ago （正确）
It has been 3 years since he hasn't come here （错误）
3. 完成时不能接瞬间动词，要接持续动作，所以【他有多久没来这里了？】应用：
How long have he been out of here?
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