Therefore, as a result, so, then, consequently
She was therefore unable to find a solution.
So she had to quit her job.
I think; therefore I am.
‘We have missed the train.’ ‘Then we are going to have to hire a taxi.’
They grew up in Japan; hence their interest in Zen Buddhism.
He sustained severe injuries and, consequently, spent the rest of his life in a wheel chair.
Therefore is common in mathematical proofs.
笔记：Therefore 常用于数学证明。
So and then
There is a difference between then and so. Then is used when one speaker replies to another. So can be used in both ways.
‘The last bus has gone.’ ‘So/ then we will have to walk.’ (Two speakers)
‘The last bus has gone, so we will have to walk. (NOT ‘The last bus has gone, then we will have to walk’) (Same speaker)
笔记：一个人说话另一个接话，用then。一个人说话，接着说出结论用so
Hence and thus
Hence and thus have the same basic meaning and are often interchangeable. However, there is a slight difference. Hence usually refers to the future. Thus usually refers to the past. It is often used to indicate a conclusion.
Both sides played well, thus no winner was declared.
The situation is getting more and more complicated. Hence we will have to proceed with caution. (NOT Thus we will have to proceed with caution.)
笔记：Hence 用于总结将来可能发生的事情，Thus 用于总结过去发生的事情。