The United States Catholic Catechism for Adults offers a take on the three elements characterizing moral acts that may be a bit easier to grasp than the Catechism of the Catholic Church's presentation.
Under "Life in Christ/The Foundations of Christian Morality/We are Moral Beings: Fundamental Elements of Christian Morality," we find the subsection "The Understanding of Moral Acts":
Another important foundation of Christian morality is the understanding of moral acts. Every moral act consists of three parts: the objective act (what we do), the subjective goal or intention (why we do the act), and the concrete situation or circumstances in which we perform the act (where, when, how, with whom, the consequences, etc.)
For an individual act to be morally good, the object, or what we are doing, must be objectively good. Some acts, apart from the intention or reason for doing them, are always wrong because they go against a fundamental or basic human good that ought never to be compromised. Direct killing of the innocent, torture, and rape are examples of acts that are always wrong. Such acts are referred to as intrinsically evil acts, meaning that they are wrong in themselves, apart from the reason they are done or the circumstances surrounding them.
The goal, end, or intention is the part of the moral act that lies within the person. For this reason, we say that the intention is the subjective element of the moral act. For an act to be morally good, one's intention must be good. If we are motivated to do something by a bad intention -- even something that is objectively good -- our action is morally evil. It must also be recognized that a good intention cannot make a bad action (something intrinsically evil) good. We can never do something wrong or evil in order to bring about a good. This is the meaning of the saying, "the end does not justify the means." (cf. CCC, nos. 1749-1761)
The circumstances and the consequences of the act make up the third element of moral action. These are secondary to the evaluation of a moral act in that they contribute to increasing or decreasing the goodness or badness of the act. In addition, the circumstances may affect one's personal responsibility for the act. All three aspects must be good -- the objective act, the subjective intention, and the circumstances -- in order to have a morally good act.
This teaching, which recognizes both the objective and subjective dimension of morality, is often at odds with a perspective that views morality as a completely personal or merely subjective reality. In such a view, held by some in our culture, there are no objective norms capable of demanding our moral compliance. Such a denial of an objective and unchanging moral order established by God results in a vision of morality and moral norms as being a matter of personal opinion or as established only through the consent of the individual members of society.
What I like about this way of putting it is its relative simplicity. "What we do" and "why we do it" are things we're used to thinking about. And emphasizing "both the objective and subjective dimension of morality" is important these days, as the Catechism says.
The price of simplicity is, in part, that it leaves unspoken how to decide what "what we do" is. But at least stating the independent objectivity of what we do shows that it is not wholly fungible, and hints at the possibility that what we say we are doing, or even what we think we are doing, isn't objectively the case.