Here's why:
* Stem changes: The stem of the verb changes in some conjugations. For example, the stem changes from "viv-" to "viv-" in the present tense (vivo, vives, vive, vivimos, vivís, viven).
* Irregular yo form: The "yo" form of the present tense (vivo) does not follow the regular -ar pattern (vivar).
Let me know if you would like to see the conjugations of vivir!