The first article asks whether individual, existing, material things can be causative agents in their own right, or whether some more universal Form or Idea acts through them. The import of this is whether you learn about things merely by pondering them, or by studying the behavior of concrete examples. St. Thomas decides that individual things are causative agents in their own right, which means that you need to study chemicals (and see what they actually do!) to learn chemistry. Some areas of philosophy are armchair activities, but some are not. However, this also means that studying things in this fashion has a limitation. By studying things as individuals, precisely in their quantitative aspects, you are not studying the non-quantitative aspects, which give us abstract, universal knowledge.
The second article is about whether the material world contains implicitly within it the power to bring forth new things. St. Thomas' answer is yes, just because something is not manifest does not mean it is not possible. See post on ID.
I like James Chastek's comment on articles three and four. At first glance, St. Thomas seems to be referring to astrology here. However, that is really not his style. What St. Thomas really means is, being corporeal creatures, we are affected by material things. Some material things are more fundamental than others. For St. Thomas, those things were called celestial bodies. Stars and such were supposed to be more pure than earthly things, to embody whatever they are more clearly. This is clearly untrue, but the idea is that simpler, purer material things can be see to be the cause of more complex things. Today, we would talk about atoms, or energy, rather than celestial bodies, but the principle is the same.