[Natsumi]

"It would be better whether these are fully equipped tours"

"Whether" is incorrect in this sentence. The correct word would be "if", but I don't understand why. I know that whether is more formal than if, but in this sentence it doesn't sound right.
Could anyone give me an explanation?

Thank you in advanced.

[susan53]

If has three uses - it can express : a) a condition, b) the idea "given that X is true" or c) the concept possibly yes, possibly no

Whether doesn't just mean if - it means if or not. So it can only be used in contexts where there is a yes or no possibility - ie replacing if when it is used with its third use. Some examples :

I don't know if/whether I shall go to the conference (possibly yes, possibly no)
He asked me if/whether I liked cats (possibly yes, possibly no)

In your context there is no yes/no possibility. Here if expresses a condition - not a yes/no alternative, and therefore whether cannot be used. Notice that it's a hypothetical condition (currently the tours are not fully equipped) and so the second form of the verb should be used :
It would be better if these were fully equipped tours

Other examples where if expresses a condition and therefore can't be replaced by whether include :

If the phone rings while I'm gone, would someone answer it please?
If I hadn't missed the plane, I'd be on the beach now!


And some where it means Given that X is true... :
If you saw him, why didn't you tell him about the meeting?
If you knew about it, you should have told me.
etc

[Natsumi]

Thank you Susan53. Your explanation has been very useful to me. I've always been taught that whether is formal for if, and clearly that's not true.