You will find an example of a sound and unsound deductive argument here.
A sound deductive argument would be:
All squares are rectangles.
A rectangle has four sides.
All squares have four sides.
This is sound because the definition of a square and rectangle both include that element of four sides.
An unsound argument would be:
All toads are cold-blooded.
All reptiles are cold-blooded.
All toads are reptiles.
In this case, toads and reptiles do share the same characteristics, but a toad is not a reptile. It is an amphibian.