从今以后如果你能快乐

If you should be happy from now on

should可以用在if引导的条件从句,表示一件事听起来可能性很小,但也不是完全没有可能,表示为虚拟语气

If you'll be happy in the future.
If you'll be happy from now on.

if you will be happy from now on..

if you could be very happy from now on

Since now,if you are happy~~~~~~~