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when習慣上不與現在完成時連用

■when 表示“何時”“什么時候”時，可用于過去時態或將來時態，但通常不用于現在完成時。如：

你什么時候和他首次見面的?

誤：When have you first met him?

正：When did you first meet him?

■但是，since when 可以與現在完成時連用。如：

Since when have you lived here? 你從什么時候起住在這里的?

Since when have you become active? 你什么時候開始變得積極了?

Since when do you read newspapers? 你什么時候開始讀報的?

That was in 1949, since when things have been better. 那時是1949年，從那以后情況就好些了。