Are these sentences different or same?
They don't let you smoke in here.
They won't let you smoke in here.
Also, is it necessary to use in in the sentences?
Would the meaning of sentences change if I do not use "in"? Can't I just say "They don't let you smoke here."?
Best Answer
I partially disagree with the two other answers (as of the time of this writing).
The Don't.
Let's use a slightly different example sentence to make the nuances clearer:
This means that the classmates have a general habit of not following the rules. It's form of time-agnostic knowledge; it's possible that you're now attending another school and talking about ex-classmates.
The Won't.
This suggests that there exists effort directed towards making the classmates follow rules. The phrasing gives an implication that there's an opposing force that's trying to behave your classmates.
Your example.
This is general; the speaker has probably deduced that from a no-smoking sign or similar.
This would imply that the speaker has probably actually tried to smoke once and they didn't let him. You might end up getting the implication that he tried to convince them ('oh, just this one cigarette'), which is the effort we mentioned in our conceptualization.
Note: this is a pretty small and nuanced difference; nobody will get confused if you use them interchangeably, but it does come quite naturally for native speakers.
Lastly: the In.
Simple:
This is localized; it was probably getting crazy in a room, hall, or some other enclosed 'cozy' place.
This is more general and can refer to any type of place that is more likely abstractly-bordered-and-contained than having brick-and-mortar walls; think countries and nations, school campuses, streets, etc.