therefore有两种用法,一在整句中,一在分句中.therefore是副词,但有时具有连词作用.
一、整句中时一般不放句末,句首后要有逗号,句中1、按一般副词使用2、做插入语（这两个分得清吧）
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.