لا يهم: it doesn't matter لا مهم: it actually should be corrected to "ليس مهما" which means: it's not important

there is difference between مهم .... يهم مهم : صفة (adjective ) يهم : فعل ( verb ) we say : لا يهم means : It does not matter we say : غير مهم or ليس مهم و ليس مهما means : unimportant. we don't say : لا مهم