" 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 ?
The meaning is slightly different.
The "have still to" form means that the action is required and as yet not performed.
The "still have to" form means that the requirement for the action is still in force. It does not say whether the action has been performed in the current instance.
Example: You cannot go to your party yet. You have still to clean your room.
Your room must be cleaned before you go to the party. You have not yet done so.
Example: Now that you have reached the age of 16, you have some additional privileges. But you still have to clean your room before you go to any party.
The requirement to clean your room has not been removed. There may or may not be a particular party being discussed at this time.