One of the most difficult things I find in the course are the Venn diagrams. Those dealing with universals are much easier than those dealing with existentials. The problem with existentials is that you cannot pin them down anywhere you can just say that the only sure thing you know is that there are some in a certain area. There are fleeting entities. You can be sure some are in an area but what about elsewhere.

Universals are much easier. You can pin them down to only certain areas. In this way you can definitely say whether expressions are valid or invalid. Just point where one is and the other is not. Even with expressions difficult to pin down one cam always De Morgan them or apply some other transformation. p->Q is an example. It is much easier to treat this as ~PvQ on a Venn diagram.

The way to deal with existentials 'seems' to be by showing counter examples to the expressions. This may make sense from a conceptual point of view but when you actually doing it  feels like cheating. One comes up with one's own conditions - one encapsulates the universe by coming up with a set that helps one give an answer to the problem of validity. One is always left with the thought: "Am I being honest about the set I am creating or is it just a set of convenience"