It’s more a matter of what the Church and/or the Bible says is mortal sin. Something like telling a quick lie, with not thought of it in advance, to get out of a difficult situation wouldn’t be a mortal sin. Like, if you meet someone you know while out shopping and this person starts to really start talking, and you don’t have time to stand there and listen to their whole speel, you say something to get away from them. You might not want to say to their face that they are being rude. But make up some excuse in order to go on to what you are trying to do. If you say something like, “I have to hurry, I have a doctor’s appointment in half an hour.” That wouldn’t be a mortal sin. At least, if not really thought out ahead of time. But if you go out and kill a bunch of people, then that would be a mortal sin. But if you’re still unsure, talk with your priest. He may be able to give you to help discern between what is mortal sin and what is venial sin.