'it' in infinitive construction

[Pokemon]

Would you please tell me if the pronoun 'it' is:
a) optional
b) mandatory
c) a mistake

in the following sentence:

This article is too difficult for me to translate (it).

[Cerberus™]

Normally it is a mistake in this type of sentence, which is:

Subject (the article)
+ adjective belonging to subject or subject complement (too difficult)
+ infinitive belonging to adjective (to translate)
+ object of the infinitive, which is identical to the subject [the article?].

In this case, the object is simply left out, no "it":
This article is too difficult for me to translate.

But "it" should be added if: a.) the subject of the sentence and the object of the infinitive are not identical; or b.) if the construction is not as described above; or c.) leaving "it" out would result in confusion. Note that "it" may lead to confusion, too, in which case the entire word is best repeated.

a. The main pillar of this bridge is too weak to support it.
b. This article is so difficult that I cannot translate it.

[Pokemon]

Thank you very much, Cerberus. Does the same rule work with "This article is easy enough for me to translate"?

[Cerberus™]

Yes, although perhaps the "rule" is a bit less strict when there is no "too" involved, I guess it is a bit fuzzy. Even so, leaving "it" out of this sentence is better under normal circumstances. It also depends on context a bit: there might be a situation where you'd want to emphasize the object, in which case you could add "it".

[Pokemon]

Thanks a lot, Cerberus.