Copular sentences are different from others. They normally begin with the subject followed by the predicate marker, then predicate. There is no form of the verb to be in Umu.
Even simpler sentences come without a subject.
To explain how the predicate marker behaves in this situation, some background information is necessary.
Copular sentences omit subject markers, but they are still assumed to be there. In order for the copula to equate things properly i.e. (A=B), or (A is B) or (this is that), the subject marker and predicate marker must agree. This is why, in the copula, the predicate marker actually agrees with the subject.
Words that modify the subject come before the predicate marker, just as they would in regular sentences.