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"，在这里比喻心情"沉重的"。 heart 本义是"心脏"，引申为"心情、情感"的核心。 with a heavy heart 是固定搭配，字面意思是"带着一颗沉重的心"，实际表示"怀着沉重的心情"。