Whenever we substitute a letter for a term in an argument, we must do so consistently throughout the entire argument. Thus, if we substitute 'A' for 'cats' in line one of example 1, we must do so again in line three. So arguments may be structured in the same form you say. If an argument has a valid form, then any substitution instance of that form will also be valid. In other words, the validity of an argument is guaranteed by its form and does not depend on its content.
All collies are dogs.
Some animals are not dogs. We have already encountered such examples in the text. All citizens of Montana are Americans. So, all citizens of Nebraska are citizens of Montana. As we discussed, this is an invalid argument.