"They" represents more than one person. More than one person cannot wear just one dress or shirt. Well, I suppose they could, but that would be a discussion for a very different forum........
The last sentence you listed, "They wear a new shirts," is even more improper. The article "a" refers to a single item, so when you pair it with the plural "shirts," it does not agree.
Therefore, the two correct sentences listed are: "They wear new dresses." and "They wear new shirts."
|link||answered Mar 08 '12 at 13:29 Erik Czerwin Contributor|
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