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同学你好，
therefore放句首后要有逗号，放句中做插入语后无逗号。
eg.1、Therefore，we must learn English well.
2、They therefore can learn English well.

therefore有两种用法，一在整句中，一在分句中。
一、整句中句首后要有逗号，如句1、按一般副词使用没有逗号如句2、做插入语则有逗号如句3。
eg.1、Therefore，we must learn English well.
2、They therefore can learn English well.
3、Many fast food restaurant ,therefore,have red furniture or walls.

二、用在分句中，即一个句子一部分表示原因一部分表示结果。
这时一般词前要用分号，其后用不用逗号无所谓。若第二个分句前是逗号或无符号，则要注意therefore是副词，和so不一样，要保持句子完整，应用and therefore。
eg.1、I had a headache;therefore I could not go to your party.
2、I was ill,and therefore could not come.
3、These birds are very beautiful and therefore liked by many people.

祝学习愉快