Can we say
"I will be leave on this Monday."
Is this sentence grammatically correct?
Or we have to say
"I am going for a leave on this Monday."
It depends if leave is being used as a verb or a noun.
If leave is being used as a verb, then the right way to say this would be something like one of these:
I will be leaving on Monday.
I am leaving Monday.
I will leave this Monday.
(Notice how we use leaving when a form of be is preceding the verb.)
Sometimes leave can be used as a noun, though. In some job situations, the expression on leave means "having an excused (or paid) absence from work". We might use this expression similar to how we would say "on vacation".
To use leave as a noun, we would say something like:
I am going on leave starting Monday.
I am starting leave on Monday.
I will be taking leave on Monday.
Neither "I will be leave on this Monday." nor "I am going for a leave on this Monday." is correct.
You can say :
1) I will leave this Monday.
2) I am leaving on Monday.
You can say this way when you are talking about arrangements.An arrangement is is a plan for the future that you have already thought about and discussed with someone else.
3) I am going on leave from this Monday.
It's like the previous one. But here "leave " is used as a noun.