Finding the "second character"?
已 — 已经
  易 — 容易


You can look up these word in a dictionary.


  Like 开 (for water) how can I know it's 烧开...I don't know that a dictionary will tell me that


开 is called a resultative complement (结果补语). Unfortunately, there is no easy way to know them.

Most adjectives can be used as resultative complements. e.g. you can say 水开了 and 烧水, so you say 水烧开了, too.

But most resultative complements are not adjectives. You have to learn them as collocations. e.g. 


  看见/看到
  听见/听到
  but 闻到 and 想到. 闻见 and 想见 are relatively rare
  找到/找着


You may have to ask the teacher or use Google to find out which one is more common. e.g.

If you search "看不*东西" (including the quotation marks) in Google, you can only find 看不见, 看不清 and 看不到.

Technically, 到 in 看不到 is a potential complement (可能补语). But in most cases, resultative complements are the same as potential complements. The main difference is that you put 得 or 不 between the verb and the complement. This trick can help you eliminate many false positives.