I can't imagine such a passion would be so plain,steady and endyring!I can't believe a passion can be so down to earth, stable and everlasting.
PS: Plain is a neutural or negative word thus it does not fit this sentence. Steady is used for objects more often. Endyring is spelled wrong, and it should be enduring. It is often used for a long period of suffering pain, not good at all here.
I can not believe a affection can as this flat dependable lasting
It is hardly to believe so smooth,stable,and everlasting the affection is.
No one can keep his feeling ocean peace long without a storm.
