It seems you might have some large misunderstandings about the way Hammer operates.
First of all, Hammer doesn't autodetect anything, especially not mod folders, so whatever it was doing, it was what you told it to do. Second of all, Hammer never looks for sentences.txt, that's not its job. Sentences.txt exists only to store the names and file locations of audio files so they can be quickly accessed by the game engine. When a character is told to speak something, the engine references sentences.txt so it immediately knows where to look. Hammer has nothing to do with this.
I'm also not entirely sure what you're trying to do with this mod. I sort of get the impression that this is an Opposing Force mod, but if you're simply trying to get Opposing Force content to work within Half-Life as a mod, that too is possible. For that though, you will require the entire contents of Opposing Force's PAK file to be extracted into your mod folder, because regular Half-Life doesn't contain that stuff. If it's just a mod for Opposing Force though, you won't need any of the redundant content you extracted from its PAK.
Oh, and by the way, if you permanently deleted your PAK file from the Gearbox folder, you're kinda screwed. That's the entirety of the game files right there.