As I mentioned before, St. Thomas follows Aristotle in regarding the final, efficient, and material causes of an act -- the "why," the "who," the "by what aids," and the "about what" -- as circumstances of the act. Which is to say, he sees these causes as accidents, things that could be otherwise without changing the substance of the act.
Hold it right there, says Objection 3 of ST I-II, 7, 3. What do you mean, the causes of the act don't change the substance? The person who performs the act is, as you say, the principal efficient cause, and you're trying to tell me the person who performs the act doesn't affect the substance of the act? So me spending my money and you spending my money are substantially the same act? What kind of cockamamie theory of human acts is this? (I develop the objection somewhat beyond its written form.)
In reply, St. Thomas allows that there can be conditions of the various causes that do affect the substance of the act, and that these conditions are not circumstances. Whose money is being spent, for example, is a condition of the act, not a circumstance, since it can change the act from "use of one's own property" to "theft"; the circumstance in this case would be whether a lot or a little money is being spent.
That said, the various causes can and do give rise to circumstances of acts that don't change the substance. When the act is "use of one's own property," the "who" is constrained to the owner, but when the act is "theft," the "who" could be pretty much anyone else and it would still be theft.
The final cause always has at least one condition associated with it: the object of the act, which is to say the proximate end sought that determines what kind of act is being performed. Remoter ends, though, are always circumstances, because they can't modify the substance of the act itself. This is why knowing that a person has done something only tells you that he wanted to do it, not why he wanted to.
As lagniappe, St. Thomas adds that the effect of the act, which brings in the circumstance "what,"* can also include conditions that change the substance of the act. The example he uses is of washing someone with water: it's a condition of the act of washing someone that you wash them by the act of pouring water over them. Other effects -- he catches a chill, he gets scalded, and so forth -- are circumstances.
* I think "what is effected" might be clearer in meaning, if less precise a translation of "quid agitur" than the "what is done" the standard English translation of the Summa uses.