There are several verbs in Latin that mean the same or about the same as the BE verb in English. The most common one in usage is ESSE which we will explore here in the present tense.
Like most Copula verbs in most languages, ESSE is irregular however it is still very simple to learn.
Sum means "I am".
Es means "You are".
Est means "He is".
If you are familiar with older English this should be easy to see the link. We used to say;
Now the M in the Am is fine, we can just link that in our minds to the M in Sum.
As we see above, in older English the letter T was for the You form and the S was for the He form. In Latin this is the reverse. The S characterizes the You form and the T characterizes the He form.
Now in English we have just one form, Are, to take care of all plural forms. In Latin however, we need a different form for each.
Sumus means "We are".
Estis means "Ya'll are".
Sunt means "They are".
You should notice that while in English we use "you are" for both singular and plural; in Latin we use two different forms. English only has a total of three forms, Latin has a total of Six. You should also notice that while we do use a different form for the plural, it is similar to the singular form. M changes to MUS, S changes to TIS, and T changes to NT. See that isn't hard at all.
Now if we consider the fact that the original base of the verb was ES, and we attempt to add our endings to just a bare ES we get this,
If you try to say these as is you will find yourself automatically inserting an "oo" sound between the S and the M or N. Eventually that U sound got too big and it drowned out the E sound at the beginning and people stopped saying it. The only other issue here has to do with ESS which would just be a double S sound that people would probably been happy to just say a simple ES and move on with their lives. Thus in a very natural way we then get;
You should also make sure to remember these endings, -M, -S, -T, -MUS, -TIS, and -NT. You will see these over and over again. The first person M is more often an O in most verbs in the present tense but will become M again in other tenses.