Making pronouns agree in number with their antecedents is a tricky business. The following sentence has an agreement problem because the word writer is singular and their is plural:
A good writer knows how to punctuate their sentences.
Here are three fixes, from least to most favored:
1. Use his or her to agree with writer:
A good writer knows how to punctuate his or her sentences.
2. Make the noun plural to agree with the plural pronoun:
Good writers know how to punctuate their sentences.
3. Best of all, delete the pronoun in the name of conciseness:
A good writer knows how to punctuate.
To purchase your copy of The Art of On-the-Job Writing by Philip Vassallo, click here: https://www.firstbooks.com/product_info.php?cPath=14&products_id=144
To purchase your copy of The Art of E-Mail Writing by Philip Vassallo, click here: https://www.firstbooks.com/product_info.php/cPath/53/products_id/196
