Can I use "therefore", "so", "hence" and "thus" interchangeably?

Solution 1:

Therefore is an adverb, as well as hence and thus, although used in this similar manner they are conjunctive because they denote causal relation between the two clauses in your statement.

Both of your examples above are acceptable, but there should be a semicolon in your first therefore example:

I was tired; therefore, I fell asleep.

Solution 2:

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 do what we said."
• Thus should indicate the past or indicate a conclusion, such as "They couldn't see eye to eye, thus they didn't decide anything."