if Soul Eater Not! was supposed to happen before the original Soul Eater series. Than what happened to Tsugumi Harudori,Anya Hepburn and Meme Tatane? How come they don't appear in the original Soul Eater series like everyone else does?

( If there is an incorrect fact in my answer, please feel free to correct it. I read and watch Soul Eater and Soul Eater Not!, but I have a short-termed memory, so I don't really remember the important fact.)
Anya, Meme, and Tsugumi are from the NOT class. Maka and the others are from the EAT class. EAT class envolves the student to undertake various missions to go out onto the field and reclaim tainted souls along with collecting 99 evil human and 1 witch souls in total. NOT class is for people or weapons who want to control their weapon abilities but do not want to risk their lives on the battlefield, being in NOT is strictly for mastering the basics and maturing their individual talents. EAT and NOT class are very different, so the people in the EAT and NOT class have separate ways, therefore you don't see a NOT student in Soul Eater.
If you really want to see a Soul Eater Not! character in the Soul Eater series, Tsugumi appears in Chapter 99 of the Soul Eater manga. ( If you still don't understand it, I'm sorry. I tried my very best).

Your right,meow!!!!!