You can't smoke (here / in here)

Question

You can't smoke (here / in here).
What's the need of adding in before here here.
Can't only here suffice.

Answer 1

Both are correct, but there are subtle differences between both expressions.

'here' simply implies a broader sense of location, and that location is usually implicit in the sentence.

You can't smoke here.

implies that the person can't smoke in the same location as the speaker, wherever they happen to be.

'in here' on the other hand, narrows the scope somewhat, because it refers to a space that can be entered, such as a room or building. Other prepositions that work, depending on context, are 'out', 'down', 'up'. For example:

You can't smoke in here.

You can't smoke out here.

You can't smoke up here.

You can't smoke down here.

Answer 2

In is added only if the place in question is an enclosed one. That way, it adds weight to the fact that it is, in fact, a closed space. For instance, a room.

If it is an open space, like a ground, in should be omitted.

Answer 3

Yes, only using here will suffice. As stated, in refers to an enclosed area. In might be used to imply that smoking is allowed nearby.