Which of the following two sentences sounds more natural, please!
#1 My mother always tells me to tell the truth when I tell a lie.
#2 My mother always tells me to tell the truth when I'm telling a lie.
My mother always tells me "When you lie, tell the truth" or "When you are telling a lie, tell the truth".
Your intended meaning (I think) is:
Whenever I tell a lie, my mother tells me to tell the truth.
Whenever I'm telling a lie, my mother tells me to tell the truth.
The first sounds better.
"If my mother catches me lying, she always reminds me to tell the truth."
Naturally the suggestions of Raymott and konungursvia grant more clearness to the original sentece. But maybe Heidi's #1 sentence is meant to repeat "tell" three times in purpose, like saying something fun, urging the listener to think deeper about it. Of course if that is the idea, the sentence is not supposed to sound so "natural".