It was with a heavy heart that he packed his bags and left his home of many years.

with 是介词，表示"带着、伴随"某种状态或情感。 a 是不定冠词，表示"一个"或"某种"。 heavy 本义是"重的"，这里比喻为"心情沉重的"。 heart 本义是"心脏"，引申为"心情、情感"。 with a heavy heart 是固定搭配，表示"怀着沉重的心情"，形容人在做某事时内心感到悲伤或不舍。