So recently I have been using my phone for entertainment and then my mother suddenly entered the room and asked if I have already done my assignment. Out of fear from being scolded at I told her that I already finished my assignment. Then I told her the truth and all but did I commit a mortal sin? I think not because the act was not that grave. But I still want to ask
Ask your Priest.
It’s generally a bad idea to ask strangers on the internet these things. I would suggest finding a priest to ask.
I often direct people to St. Thomas Aquinas when they ask these questions, as he answers them pretty clearly if you’re able to parse his writing.
Here’s what he says: Aquinas on lying. He says that lying about some contingent fact not really relevant to the person you lied to wouldn’t normally constitute mortal sin unless you do so with malicious content.
It’s great that you thought again and told your mother the truth.
To me the fact you’re asking means you wouldn’t have had full knowledge and therefore it was not mortal. Best to get advice from a priest.
Some lies would be though, like perjury or lies that cause real harm to others.
I won’t weigh on on mortal vs. veniel.
But I will tell you, don’t get in the habit of lying. Once it becomes a habit, I have watched how hard it is to break. I really believe it is probably the worst habit there is.
It is so good that you caught yourself and then told the truth. That shows great integrity, even if you did lie to begin with. Stay on that path. We all make mistakes. How we recover and correct is the mark of what kind of person we are.