I have tried to make sense of this type of construction and I think I've just reached my limit. I have to say I can't accept the first sentence in the first statement. Could you explain why the tense inconsistency between the sentences in the first statement is possible?
Nobody can unless he has 10 arms and (10) legs. - This is a statement which, in any case, is impossible as nobody will have 10 arms and legs.
Nobody can speak another language unless they are prepared to spend the time learning it. (- it is possible that they will spend the necessary time.) They would have to commit to a course of lessons, lasting mon