Sometimes, learning grammar rules can be torturous. Or is it tortuous?
Be careful with these two words; the second -r makes all the difference. Torturous has torture as its root, and it means that something is very painful, that is, causes torture. Having to sit through a 45-minute meeting was torturous.
Tortuous (without the second -r) refers to something that has many twists and turns, as in Malibu Canyon is a tortuous road. Tortuous also can refer to something that has many twists and turns psychologically, that is, is circuitous, as in His argument was so tortuous that I never quite saw his point.
So, is learning grammar rules torturous or tortuous? I suppose depending on who’s doing the teaching, it can be both.
Sherry
Everything Language and Grammar 
And don’t forget tortious, from the root tort, meaning a wrongful act which could subject the perpetrator to civil liability. I’m sure there are lawyers who could think up a theory by which learning grammar rules could be tortious.