Whether the label before a contributor’s name–for example, edited by or translated by–is capitalized or lowercase depends on its position in the MLA format template.

In the following example, the label is capitalized because the contributor’s name follows the “Title of source” slot. Since the title of a source is always followed by a period, the label before the contributor’s name must be capitalized:

Cervantes, Miguel de. Don Quijote. Translated by Burton Raffel, W. W. Norton, 1999.

In the example below, the label is lowercase because the contributor’s name follows the “Title of container” slot. Since the title of a container is always followed by a comma, the label before the contributor’s name must be lowercase:

Dostoevsky, Fyodor. Notes from the Underground. Notes from the Underground and The Gambler, translated by Jane Kentish, Oxford UP, 2008, pp. 1–124.

In other words, don’t capitalize labels before contributors’ names unless they follow a period or consist of words that would normally be capitalized.