Sin seems to be too easy to comprehend but in complicated circumstances one will find it hard to know whether an act is sinful.
The magic word in understanding sin is RELATIONSHIP. In fact the word sin in it.
Here is a typical example. A young couple got married because of the pressure from their families for the woman has gotten pregnant. Both of them were not yet ready emotionally and financially but to protect the dignity of the woman's family the couple was forced to enter into marriage.
As expected, their marriage failed and eventually they separated. Having a life of their own now, and has grown more maturely they found new loves and are now emotionally ready to take the responsibility of husband and wife and as parents. But since their previous marriage was valid then civil and canon law wise, they are not eligible to marry. Result, both were living with their respective partners without the blessing of the sacrament of matrimony.
Question, are their acts sinful or not?
Conventionally it sinful but for the persons involved, they can rationalize that they are not doing anything evil. All they knew is that they truly love and that loving is itself good. Had it not been to their families pressure, then perhaps they did not marry in the first place.
To resolve the issue, the question that needed to be asked is: Is the act breaking relationship or not. if it does, then it is sin if not then it is not sin.
If you read closely, the ten commandments are all about relationship. Relationship to God, parents and neighbours.
