Here’s a sentence with a pronoun problem that my husband and I found in the draft of a gardening column he was writing:

Mulch heavily and add to it annually.  INDEFINITE PRONOUN REFERENCE

The it is an indefinite pronoun reference. It doesn’t refer to a specific thing earlier in the sentence.

This might seem puzzling: “It” clearly refers to mulch, and there’s the word m-u-l-c-h four words earlier in the sentence:

Mulch heavily and add to it annually.

Nice try, but wrong. Mulch in this sentence is a verb (action), not a thing.

Here’s how we corrected the sentence:

Apply mulch heavily, and add to it annually.  CORRECT