A pronoun should never refer to more than one noun phrase. Whenever a pronoun may refer to more than one noun phrase, your reader will be confused.
What is confusing about this sentence?
The passengers waited all night while the airplane mechanic and her assistant worked to repair the engine. By morning, they were exhausted.
The reader cannot tell whether ‘they’ refer to ‘passengers’ or ‘airplane’ ‘mechanic’ and ‘her assistant.’ The use of the pronoun ‘they’ is said to be ambiguous.
What is confusing about these sentences?
The cat knocked over the table with the lamp on it, and it broke.
Ned was riding his horse over the bridge when it fell into the water with him on it.
There are too many books on those shelves. If you are not careful, they’ll fall.
Watch out for ambiguous pronouns!
Any time you use a pronoun to replace a noun phrase, make sure the pronoun refers only to that noun phrase and not to some other one as well. An experienced writer will check every pronoun to be sure that it refers to just a single noun phrase.
Are there any ambiguous pronouns in these sentences? Which pronouns are they? How would you rewrite these sentences so that they are no longer ambiguous?
 There were many people in the restaurant tonight and the waiters worked very hard. They were tired when they got home.
I got a letter today from an old friend, and I was pleased to hear from her.
 There were so many people that it was hard to move and so many different refreshments that I couldn’t choose. They were all gone by morning, though.
 Judy and Marcia, who were running for the student council, went to watch the school basketball team. They won that night.
 While Fred was walking his dog, he broke his leg.
Always be sure to check your writing for ambiguous pronouns. They can appear when you are least expecting it. Sometimes thoughts that are clear to the writer’s mind are not so clear on paper.