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I was taught that, at least, 'therefore' and 'so' and can be used interchangeably, one being informal, the other formal. But, even when written, replacing 'so' with 'therefore' doesn't seem correct.

I was tired so I fell asleep.

...

I was tired therefore I fell asleep.

Am I even allowed to use therefore as a conjunction here? The dictionary says I can, but it would seem more suitable to say:

I was tired and therefore I fell asleep.

I realise hence and thus (and even ergo) are rarely used, but where do they fit into this?

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"Therefore" is used in argumentation rather than trivial or descriptive contexts. If you are trying to convince someone with arguments, you would use "therefore"; in the sleep example, "so" fits better. Note that there should at least be a comma before "therefore". –  Cerberus Jan 8 '11 at 3:26
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You may wish to correct the spelling of the word "interchangeably" in your title. –  Jay Jan 8 '11 at 4:21
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Actually, in the sleep example, I would skip the "therefore" entirely and just use "and". But then maybe I'm just tired... –  SamB Feb 11 '11 at 6:25
    
You can use the word "ergo" –  909 Niklas Aug 14 '11 at 10:45
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2 Answers

up vote 6 down vote accepted

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.

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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."
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