There's a problem I don't understand why I can't use absolutely should according to the book (Oxford Grammar Intermediate by Swan). Anyway, here's the problem:
Put in the best word: must or should
You absolutely (must/should) check the tyres before you take the car
out today.
The key answer is must.
What I've known, must is stronger than should. The problem is, if we put an adjective behind either of them which makes it stronger, do they still have a difference?
By the way, I've checked the usage of both of these phrases here. For the convenient reason, here's the screenshot of the diagram:
As you can see there, absolutely should is more common than absolutely must. Does that mean, the book is incorrect? I mean, the answer should be both are correct. No?
Best Answer
Your NGram graph contains a small but very important mistake. You spelt absolutely with a capital A for must, but not for should. If you make both instances of absolutely begin with a lowercase a, you get a completely different story.
must implies a strong obligation, and absolutely must changes it to an unbreakable obligation.
should is more of a suggestion- the correct thing to do, though there is no obligation at all to do the correct thing. absolutely should converts it to... an unbreakable suggestion?
I don't think there is anything grammatically wrong with absolutely should and, as the NGram graphs show, it is used but, in my opinion, there is a semantic problem: absolutely doesn't really fit with a suggestion. You can say that something is absolutely black, but I don't think you can say something is absolutely grey.