不定式 说明主语内容，具体、特定、未 来：My job is to promote the new product.

动名词 说明主语内容，一般或习惯：His job is collecting folk songs.

现在分词 说明主语的特征，令人……的： The spy story was thrilling.

过去分词 说明主语的感受或状态，感 到……的：He was thrilled at the suggestion.