Perhaps you lack familiarity with the terms used in the argument and would thus find it difficult to imagine a case in which the premises are true and the conclusion false. Fortunately, as discussed previously, validity is dependent on the form of an argument. So if one carefully discerns the form of an argument and creates a substitution instance of that form which is invalid, then one may conclude that the form is itself invalid. And for our purposes, that is enough to then declare the original argument invalid. Categorical syllogisms are arguments comprised entirely of categorical statements.
Categorical syllogisms contain two premises and one conclusion.
Note also that all categorical syllogisms have three different terms. The shaded area in a Venn Diagram indicates an empty spot. Note also that universal statements have no existential import. I.e., they make no claims with respect to the existence of the entities involved. Universal affirmative statements simply tell us that IF an entity from the S class exists, THEN it must also be a member of the P class.