Breakdown of Esa modista del barrio diseña vestidos sencillos y muy bonitos.
Questions & Answers about Esa modista del barrio diseña vestidos sencillos y muy bonitos.
Because esa has to agree with modista in gender and number.
In this sentence, the speaker is referring to a feminine modista, so esa is the correct form. Spanish demonstratives work like adjectives here, so they must match the noun: ese chico, esa chica, esos vestidos, esas faldas.
Not necessarily, but in this sentence it is feminine because of esa.
A useful point is that modista is a profession noun. In actual usage, it often refers to a woman, and modisto is also used for a man, especially for a male fashion designer or dressmaker. So here, esa modista clearly means the person is female.