it came to me then how much easier it would be if we had a range of words that express different grades of gratitude.

1）整个句子主干为it came to me then，第一个it为形式主语，真正的主语是后面的how much easier it would be if we had a range of words that express different grades of gratitude——即后面整体作主语从句。

2）这个主语从句本身又由主句+从句构成，主句为how much easier it would be，这里的it为代词，表示if ...条件成立之后出现的情况，if引导条件状语从句（if意思为“假如”）。也会有人将这里的it分析为形式主语，真正主语是if从句（个人以为这样会造成混乱，而倾向于将if表示“是否”分析为引导名词性从句，表示“假如”分析为引导状语从句）。

3）how much easier it would be原本是个感叹句句型（就像How happy he is!），在1）中所说的主语从句里作主句。

that定从就不作进一步分析了。