thereby 和 therefore 都是副词，它们被用作过渡词。它们有点像，但意义和用法不一样。
一、 thereby 是副词， 意为“因此，从而”，相当于 by that means 或 as a result of，用来宣告刚才提到的事件或行动的一个重要结果或后果，它很少用在口语中，常用在正式的写作中，例如：
Ryan studied for two straight weeks and was thereby able to get the top score.
赖安连续学习了两周，因此获得了最高分。
The students volunteered at the hospital for six months, thereby gaining a deeper insight into the medical field.
学生们在医院志愿服务了六个月，从而对医学领域有了更深的了解。
He signed the document, thereby forfeiting his right to the property.
他在文件上签字，因此丧失了对财产的权利。
The doctor took swift action, thereby saving the life of the patient.
医生迅速采取行动，挽救了病人的生命。
Rohith pointed out that this was a western tradition and was thereby alien to Indian culture.
罗希思指出，这是西方传统，因此与印度文化格格不入。
This action increased demand and thereby pushed prices up still more.
这一行动增加了需求，从而进一步推高了价格。
解析：从以上例句可以看出跟在 thereby 后面的短语都是描述第一句产生的结果。
二、therefore 也是副词，意为“因此，所以”，它的近义词有 hence, thus, consequently，通常用于引出某一推断性的结论，与前述内容有因果关系，经常出现在口语和写作中，比 therefore 更普遍，例如：
Ron injured his leg two days before the game and was therefore unable to play.
罗恩在比赛前两天腿受伤，因此无法上场。
Muscle cells need lots of fuel and therefore burn lots of calories.
肌肉细胞需要大量的燃料，因此消耗大量的卡路里。
The professor told her that her words made little sense and therefore lacked any depth.
教授告诉她，她的话毫无意义，因此缺乏深度。
They heard the warning about the landslide on the radio and therefore took another route.
他们在收音机里听到了关于山体滑坡的警告，因此走了另一条路。
解析：从以上例句可以看出跟在 therefore 后面的短语都是描述第一句产生的结果。
therefore 后面还可以跟一个句子，例如：
Any human is entitled to these rights; therefore they are universal and timeless.
任何人都有权享有这些权利，因此这些权利是普遍和永恒的。
My sister likes to listen to music, therefore she often buys CDs.
我姐姐喜欢听音乐，所以她常买唱片。