COGITO ERGO SUM
Is this the most famous piece of philosophy ever uttered? When the philosopher Descartes tried to establish a sound basis for all knowledge, he realised he could doubt the existence of his surroundings. The room, after all, could be a dream. He could doubt the existence of his own body, for the same reason. But he couldn’t doubt the existence of someone doing the doubting.
This is summed up in Cogito ergo sum. “I think therefore I am.”
The verb esse (to be) is highly irregular in Latin, as it is in most languages. You can find it on a detailed handout here, with a host of other verbs, both regular and irregular.