How do I tell whether "even if" or "unless" is right?

Either even if or unless work be correct depending on the meaning of the sentence.


I couldn't have got to the airport on time yesterday unless I had found a taxi.

This suggests that you are telling it as if finding a taxi would be the only way to reach the airport and it doesn't imply whether or not you actually did find a taxi.


I couldn't have got to the airport on time yesterday even if I had found a taxi.

This one quite strongly suggests that you didn't find a taxi. It also tells us that there was no way you could've reached the airport if you found a taxi or not.

These are the only two of the five options that make grammatical sense.



This answer is really just a mish-mash of the comments on this question, I found this to be the best explanation and thought it would be useful to have a proper answer to this.