In all dialects of English, the copula "to" be is used to state location: "I'm in Boston"; and description/equivalence "I'm America," "He's a linguist."
In MAE, this is always mandatory. In AAVE, and in many other dialects of English, not to mention other languages, different rules apply.
In MAE, there are three conjugations for the present tense: am, is, are; and two for the past tense: was, were.
In AAVE, there is only one conjugation for each tense: is for the present, and was for the past.
AAVE does not usually use the copula "to be" when making a statement in the present tense. One major exception is when used for emphasis.
AAVE does use the copula "to be" in the past tense, however there is only one form: "was." Therefore "I was," "you was," "we was," etc.
"Is" as an auxiliaryEdit
"To be" when used as an auxiliary to form the present continuous ("He is running"), follows the same rules as when used as a copula.