Refering to ideas with en/y/le

Question

As far as I understand, when refering to a phrase without repeating it, one uses "en" if it begins with "de", "y" if it begins with "à" and le/la/les otherwise.  This rule failed me on the latest B1 writing challenge:   
"One day, I'm sure [of it],"  ->  "Un jour, je le/y/en suis certain,"  (I haven't even seen the phrase that le/y/en refers to yet, so how can I chose?).    "the dead will come back to life" -> "les morts reviendront à vie".    Ok, now we know what to be sure of:  "Un jour, les morts reviendront à vie".  That doesn't start with either à or de, so I guessed that "le" was the word to use in the first sentence, but the answer says "en", and I'm confused.

Accepted Answer

Bonjour Joakim,
Your first sentence is almost correct: "en" replaces something with "de", "y" with "à" (or another preposition of place: chez, dans, etc), and le/la/les otherwise.
But it's not the phrase itself that determines which of the above - it's the grammatical construction. In this case, Je suis certain de quelque chose.
It doesn't matter what the "quelque chose" is - je suis certain has to be followed by de, which means that it and the thing I'm certain of have to be replaced by en. So "I haven't even seen the phrase that le/y/en refers to yet, so how can I chose?" doesn't matter.
Does that make sense?