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

例句：

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。
例句：

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.