" researchers have still to decide whether smell is one sense or two- one responding to odours proper and the registering odourless chemicals in the air. " Can I ask you guys why it's not still have to in this sentence ?
It's not "still have to" because, in this case, the placement of still is optional; it can go before or after the verb.
My guess the rule is it goes before the verb, but still makes sense when used after. It's like six of one and half a dozen of another.