Final answer:
The statement is true. Professor Umbridge does tell Harry to write the lines 'I must not tell lies'.
Step-by-step explanation:
The statement is true. In the Harry Potter series, Professor Umbridge does indeed tell Harry to write the lines 'I must not tell lies'. This is part of his punishment for speaking out against Umbridge and the Ministry of Magic. The lines are repeatedly written by Harry as a form of detention, etching the words into the back of his hand with a special quill.