1.Here are the things you should do to be a critical reader.

1.Here are the things you should do to be a critical reader.

before you are 18 for teenagers.是对整个句子的修饰，定语从句you should try是修饰some things。

一般来区别是修饰整个句子还是从句，你把这个句子去掉看影响不影响整句话的表达意思。如果是修饰从句的话，你去掉后不影响整个句子的意思，但是修饰整个句子的话，你去掉后句子的意思就会不同。
Here are the things you should do to be a critical reader.
比如这句，to be a critical reader是整个句子的一部分，而you should do是从句修饰the things。这句话的意思是，要做一个批判性的读者，你应该做这些事情。如果去掉以后呢，这句话就成了Here are the things。整个句子的意思都改变了。

Here are the things you should do to be a critical reader.
[ to be a critical reader] 目的状语， in order to be a ...
Here are some things you should try before you are 18.
[ before you are 18]同样是状语，时间状语修饰动词try，try的时间要在18岁前。