To err is human

Dear ziyuenlau,

I'm happy to respond. The folks at MGMAT are very smart, but in this case, I don't know that their example was the best chosen. We are used to the sounds of these phrases in English, so these expressions don't sound awkward to native speakers.
1) To err is human. (Translation of Seneca's Errare est humanum).
2) To err is human; to forgive, divine. -- Alexander Pope
3) To err is human, but it feels divine. -- Mae West
Again, these are common in modern English, and because they are so familiar, they don't seem awkward. MGMAT's point is that these do not serve as good models. in fact, almost any other infinitive in this form would sound very awkward.

For example:
To learn idioms for the GMAT SC is important. = awkward
It is important to learn idioms for the GMAT SC. = correct
To get 700+ on the GMAT on one's first try is difficult. = awkward
It is difficult to get 700+ on the GMAT on one's first try. = correct
As a general rule, the form [infinitive]"is"[adjective] is always awkward (except in the case of the familiar saying above). Instead, we have to use the empty it.

Does all this make sense?
Mike