It is written everywhere that the person is "at " work. He is "at" work. So why we can't use "on" instead of "at" there? We use on Monday ,on Tuesday etc I know this is the rule but what if we use "on" work. Why it is wrong?
I am afraid that, as with most "why" question about language, the whole of the answer is:
Because that's how English is.
There is no other answer.
In this case, the distinction arises because "work" is a physical location, while "Monday" is a point in time.
In general, the correct preposition to use is often determined by subtle distinctions in the "category" of the thing the preposition operates on. For instance, "small" units of time tend to take "on", while "large" units of time take "in":
- The clock chimes on the hour.
- I will go to work on Monday.
- I went to the beach in July.
- I will go to Europe in 2017.
For physical locations, we use "in", "on", or "at" (among other possibilities), depending on what the location is.
- I am in the store. (Emphasizing the physical structure of the store.)
- I am at the store. (Emphasizing that you are in one place rather than somewhere else.)
- I am at work. ("Work" is an idea, not necessarily any specific place.)
- I work in an office. (An office is indoors.)
- I work on a construction site. (A construction site is typically outdoors.)
And just to reassure you that English is not completely logical, you can use at with either of the last two sentences. Sometimes, a language just accepts some usage as correct while rejecting others.