answer:That’s true. It happened in French too: être étais are from Latin stare, and est sont serai are from Latin esse. In fact it happened in Latin as well; some forms of esse (fui futurus, etc) are built on a different verb. An example of when this isn’t the case is any Germanic strong verb – an “irregular” verb that has a vowel change, like sing sang sung, find found, drive drove driven etc. Many of these can be traced back to regular vowel alternations in Proto-Indo-European. Others, like catch caught, think thought, were weak (regular) verbs that became strong.