The phrase "didst a t" does not exist in the English language. Perhaps the author meant to use the word "didst," which is the second person singular past tense of "do." In this case, some antonyms for "didst" might include "didn't," "refrained from," or "avoided." However, it's important to note that antonyms are words that have opposite meanings, and "didst" doesn't really have a single opposite word. Instead, it can be replaced with a variety of other phrases that convey a different meaning. If the author had a specific alternative in mind, it would be helpful to know the context in which they intended to use it.