That was in 1949,since when things have been better.

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That was in 1949,since when things have been better.
您好!这是《张道真英语语法》第568页的句子.书中写道：“关系副词when和where也可引起非限制性定语从句.”
我的疑问是：不是说副词前面不用介词吗?例如：Come here.就不能写成：Come to here.Go there.也不能写成：Go to there.
但在这句话的since when ,since 是介词；when是副词.这如何理解?

1.少数介词是可以加副词或介宾短语等状语成分的,比如except/from/since.
2.例如：
1）I'll go abroad next year.
而：She comes from abroad.
The tiger jumped from behind the tree.
2）The article is well written except for some spelling mistakes.
3）He came here two years ago.
而：He has been here since two years ago.

I have always been interested in making things.When I was a All the things in my heart was you since I have fallen in lo A lot of things （）in China since 1978 A.was happened B.have Since when have you been in this item's segment of business I have been interested in music ever since I was a small chi l have been in shanghai since l XXX XXX（be born ）答案为什么添was Jack must have been joking when he said that he was going to 4.When he was there,he ______ have been to that coffee shop ____ have you been travelling?Since I was a boy. I have been in CZ since last year.这话对么?been in后可加since吗 I have been studying English since I was five (对since I was ____have you been travelling?since i was a boy.A Since when