We use a number of expressions to show that what is said follows logically from what was said before.

Keywords are:

Therefore, as a result, so, then, consequently

She was therefore unable to find a solution.

unable to find a solution. So she had to quit her job.

she had to quit her job. I think; therefore I am.

I am. ‘We have missed the train.’ ‘ Then we are going to have to hire a taxi.’

we are going to have to hire a taxi.’ They grew up in Japan; hence their interest in Zen Buddhism.

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.

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.