分析：主语：Americans；谓语：say；that引导一个宾语从句，还包含有一个省略了关联代词的定语从句，a person can be sure in life修饰two things. 当先行词在定语从句中作宾语时，常可省略。

Americans often say that there are only two things a person can be sure of in life: death and taxes.(*)

youdao

关系代词和关系副词引导定语从句。

Relative pronouns and adverbs introduce attributive clauses.

youdao

关系代词引导形容词从句。

A demonstrative pronoun or adjective.

youdao