This is a very good article describing what a mortal sin is. It also goes through nine of the ten commandments and breaks them down to clarify why specific sins in each commandment are mortal, as well as sins against faith, hope and charity.
This is a great way to examine your conscience before confession.
So what kind of Sins are Mortal?
In order for a sin to be mortal, it must meet three conditions:
- Mortal sin is a sin of grave matter
- Mortal sin is committed with full knowledge of the sinner
- Mortal sin is committed with deliberate consent of the sinner
This means that mortal sins cannot be done “accidentally.” A person who commits a mortal sin is one who knows that their sin is wrong, but still deliberately commits the sin anyway. This means that mortal sins are “premeditated” by the sinner and thus are truly a rejection of God’s law and love.