Some Valid and Invalid Forms

The direct way of reasoning with all...

All S are P
a is S
So a is P

All S are P + a is S = a is P


This argument is valid because it exactly the same as the argument:

All dogs bark.
Marley is a dog.
So Ralph barks.

We all know that all dogs bark and  Marley is a dog, then that means that Marley barks.

and for arguing backwards with all...

All S are P
a is P
So a is S

All S are P + a is P = a is S.

This argument would be weak because this argument is like this argument:

All dogs bark.
Marley barks.
So Marley is a dog.

This argument is overlooking possibilities. We do all know that dogs bark, but we can't jump into the conclusion of Marley is a dog even if Marley barks because Marley could be a person trying to play a title of a dog barking or something else.