You use almost to indicate that something is not completely the case but is nearly the case.
The couple had been dating for almost three years.
这对情侣谈恋爱差不多三年了。
The effect is almost impossible to describe.
这种影响几乎无法言表。
He contracted Spanish flu, which almost killed him.
他传染上了西班牙型流感,那几乎要了他的命。