The phrase "hadst a lot to do with" can be replaced by several synonyms, depending on the context and intention of the speaker or writer. Some possible alternatives include "played a significant role in," "contributed heavily to," "was instrumental in," "had a hand in," "influenced to a great extent," and "was a major factor in." These phrases all convey the idea that something or someone had an important impact on a particular outcome or situation. By using different synonyms for "hadst a lot to do with," writers and speakers can vary their phrasing and add nuance to their communication.