填 (tián) is the general verb for 'to fill' — it covers filling holes, containers, and forms. For filling in a form or document, 填写 (tiánxiě) is more specific and common; for filling a gap or vacancy, use 填补 (tiánbǔ); and for filling in blanks (e.g., on a test) or stuffing material, use 填充 (tiánchōng).