is it because llegar is irregular?
why is it "llega" instead of "llege" in the formal command of llegar?
Any verb that ends in -gar changes to -gue/-guen in the Ud./Uds. command form, in order to maintain the original pronunciation of the infinitive. The letter "g" in front of the vowels "a", "o", and "u" (hard vowels) create a hard sound; in front of "e" and "i" (soft vowels) it is pronounced with a soft sound (such as the g in "gente"). So, since the "g" in "llegar" is hard, you must use "-gue" or "-guen", for the Ud./Uds. command form, to maintain that hard sound. The "g" in "llege" would be soft, not hard like "lleGAR".
Thus: the tu command form of llegar is: "llega" (formed simply by using the el/ella/Ud. form of "llegar"), the Ud. form is "llegue", and the Uds. form is "lleguen."
Side Note: Verbs that end in "-car" change to "-que/quen" in the Ud./Uds. command form; verbs that end in "-zar", change to "-ce/cen" in the Ud./Uds. command form.
You might like to look at this link to practice/read more: http://studyspanish.com/verbs/lessons/formcomm.htm.
No llega tarde = He/she doesn't arrive late
No llegan tarde = They doesn't arrive late
No llegue tarde = Don't arrive late (?)
No lleguen tarde = Don't arrive late (?? )
but in the reference section it gave an example: "No llega/llegan tarde". Does that mean it should be "No llegue/lleguen" tarde instead of llega/llegan?