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Are the following sentences correct?
We don't know how the word was used in the 13th century, unless we could go back in time.
We don't know how the word was used in the 13th century, unless we went back in time.
Clearly, going back in time is currently an impossibility, so "unless we go back in time" doesn't seem correct. ESL grammar books haven't discussed the hypothetical (counterfactual) use of "unless" and this hypothetical use with a factual main clause.
I'd appreciate your help.
This would sound more natural to me:
We can't know how the word was used in the 13th century, unless we go back in time.
To say that you don't know something unless you do something is less successful than saying that you cannot know unless you do something. And I think go should be in the present as the verb in the main clause is in the present. However went is not wrong either, and it does put an emphasis on the fact that it is highly unlikely that we will go back. However, one must be careful with emphasis, it can distract the attention of the reader from one's main point. To my mind a sentence like
We don't know how the word was used in the 13th century, unless we went back in time.
is very ok, but just for a second there my mind lingered on went, when the main meaning seems to be contained in the first clause. Unless your intent is to focus on the fact that we cannot go to the past, I think it is alright to put go instead of went.
Having the ability to go back does not equate to doing so. So the ability alone does not let us know what word was used in that century. To find the word we have to go back. So your first sentence fails. And if we go back, we clearly have the ability to go back, so your second sentence includes the possibility that we could go back - that we have the ability to do so. Your second sentence succeeds.
