Learn Dutch: We wegen hun tekortkomingen af en klagen erover. - We weigh their shortcomings and complain about them.

Because tekortkomingen (shortcomings) are inanimate concepts, not people. In Dutch, you cannot use a preposition followed by a personal pronoun (like over ze or over hen) to refer to things. You must combine er with the preposition to form a pronominal adverb like erover.

The first verb is the separable verb afwegen. Why does af sit in the middle of the sentence right before en?

In a main clause, the separable prefix (af) gets pushed to the end of that specific clause. The conjunction en (and) connects two separate main clauses. Therefore, the first clause ends right before en, making that the correct spot for af.

Is it correct to repeat the subject and say en we klagen erover?

Yes, that is grammatically correct. However, just like in English, when two linked clauses share the exact same subject (we), you can drop it in the second clause to make the sentence flow better.

How is the noun tekortkomingen formed?