Hence and thus are by common usage interchangeable, however according to the rules of grammar they are different. Hence should indicate future use - such as "Hence we will proceed as described." Thus should indicate the past in its usage, or to indicate a conclusion, such as, "The British and American troops fought to a standstill, thus no winner was declared."
Common usage is no excuse for incorrect usage. I often interchange hence and thus when using them without thought. Tonight I heard a man say the following, "We ate like animals, we slept like animals, hence we were animals." This was incorrect, "... thus we were animals," would have been correct as cited above.
Clearly the word in question was used to indicate a conclusion, not to describe future conditions. If the speaker had said, "We will live like animals, we will sleep like animals, hence we will be animals," the use of hence would have been correct. However, he did not, thus his use was incorrect.
Thank you very much for this useful information. My students do the TOEIC test and these words are often tested. Could you recommend the grammar book where these words are explained? Many thanks for the help.