Learn English – use “therefore”, “so”, “hence” and “thus” interchangeably

conjunctionsformality

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?

Best Answer

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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