You’ve requested the example.txt file. That file doesn’t exist/exists.
Which one should I use? Why?
That file doesn't exist.
In any verb construction (auxiliaries + lexical verb) only the first is inflected for tense, person and number.
In your example does is the first verb in the construction does exist. Auxiliary do always takes an infinitive (plain) form as its complement: exist.
A note from me:
- file exists
- file doesn't exist
- files exist
- files don't exist
The right form is: The file doeSn't exist.
Gramatical explanation: always when you're forming a positive sentence about a 3rd person (singular only) in present simple tense, you have to add the letter s to the verb which describeS what he doeS.
Butwhen you're forming a negative sentence, as in your example, then this letter S goeS to the negative auxiliary verb instead to the verb itself. That's why in the sentence "The file doesn't exist" we don't add S to the verb exist, since this leeter already in the negative auxiliary verb (doeSn't rather than don't).