The phrase "had been" is commonly used to describe an ongoing action or state that started in the past and continued until another event or time in the past. Some synonyms for "had been" include "was previously," "had existed," "was ongoing," "had persisted," and "had endured." These words can be used to alter the phrase and convey a slightly different meaning or tone. For example, instead of saying "I had been working on the project for three weeks," one could say "I was previously engaged in the project for three weeks" or "I had persisted in completing the project for three weeks." Varying words can add interest and clarity to your writing.