Būt (to be) is an irregular verb. The dictionary form būt (also called "infinitive") is not used a lot, just like the word "be" in English.

In English, how do you know that you're supposed to say "I am" not "I are", "He is" not "He am", and so on? You memorize it. The same is true for Latvian:

I am = es esmu
You (singular) are = tu esi
We are = Mēs esam
You (plural) are = jūs esat

She is = viņa ir
He is = viņš ir
They (feminine) are: Viņas ir
They (masculine) are: Viņi ir

Viljams Šekspīrs: «Būt vai nebūt: Tāds ir jautājums!»