For his birthday, the large pizza was topped with all his favorite ingredients.

was 是 be 动词的过去式，在这里构成被动语态。 topped 是动词 top 的过去分词形式，top 本意是"顶部"，作动词时表示"覆盖在...上面"或"作为...的顶部"。 with 是介词，表示"用...材料/方式"。 was topped with 是被动语态结构，字面意思是"被用...覆盖在顶部"，即"上面覆盖着..."。