These expressions are often used to show that what is said follows logically from what was said before.
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 bus.’ ‘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 wheelchair.
Therefore is common in mathematical proofs.
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.)
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 train has gone.’ ‘So / then we will have to walk.’ (Two speakers)
‘The last train has gone, so we will have to walk. (NOT ‘The last train has gone, then we will have to walk’) (Same speaker)