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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". Commented Jan 8, 2011 at 3:26
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    You may wish to correct the spelling of the word "interchangeably" in your title.
    – Jay
    Commented Jan 8, 2011 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
    Commented Feb 11, 2011 at 6:25
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    As the answerers say, these words are certainly used interchangeably in many contexts. However, there are subtle shadings in their original meanings. "Therefore" started out as "therefor" and means something like "for this [reason]". "Thus" and "so" mean "in this manner". "Hence" means "from here". Commented Jul 21, 2015 at 13:59
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    This question is partially answered is a nice way here: What's the difference between “hence” and “thus”?
    – Éric
    Commented May 26, 2020 at 23:23

2 Answers 2

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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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    Semicolons are important; they serve as appropriate punctuation between two related independent clauses.
    – Ryan
    Commented Jul 31, 2015 at 22:39
  • And what's your excuse for the comma???
    – Hot Licks
    Commented Feb 9, 2020 at 15:11
  • After 'therefore'? Because 'therefore' by itself isn't a clause?
    – Ryan
    Commented Feb 10, 2020 at 19:39
  • So what's your excuse for the comma????
    – Hot Licks
    Commented Feb 10, 2020 at 21:03
  • After 'therefore'? Because 'therefore' by itself isn't a clause?
    – Ryan
    Commented Mar 1, 2020 at 10:00
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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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  • Sounds very scholarly. But have you authoritative support for these bold claims? (Oh, and Huddleston & Pullum say that the use of a comma before 'thus' at the start of a main clause is considered unacceptable.) Commented Feb 9, 2020 at 15:19

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