1. Walls have ears.
2. The walls have ears.
--------------
Which one is correct?
============================
3. Lunch is on me.
- What do you mean by "lunch is on me"?
- I mean I will pay for the lunch.
-----------------------
In the last sentence, do we have to use 'the lunch' or 'lunch'? Do we have difference between them?
1 and 2 are both correct, but 2 is the more common saying.
3 and its answers are also correct. Usually, we don't use "the" in front of "lunch," but it's not an error to do so.