Study materials
This sentence is grammatically flawless and admits no single reading.
The manager told the client that he had made a mistake.
A pronoun saves a word, but only where the reader finds its owner in a fraction of a second. With two candidates the saving turns into a loss.
A pronoun takes the nearest word
The reader attaches a pronoun to the nearest suitable noun, not to the one you had in mind. With two candidates the sentence turns ambiguous.
The manager told the client that he had made a mistake.
The manager admitted to the client that the error was his own.
There are three fixes: repeat the noun, rebuild the sentence, or move the pronoun closer to the word you mean.
What makes an antecedent accessible
The reader looks for a word that fits on three counts at once: number, gender and sense. When all three match, the link forms instantly.
The engineers submitted the report. They had worked on it for months.
So the best way of avoiding ambiguity is not a rule but a check: ask yourself how many words in the previous sentence fit your pronoun.
Distance matters
A pronoun holds its link only briefly. Two or three sentences later the reader no longer remembers what it belonged to and has to go back.
We received the report on Monday. The team reviewed the figures. The client called twice. It was finally approved on Friday.
The mistakes everyone makes
The first is leaving two possible antecedents.
The manager told the client that he was wrong. → The manager admitted that the error was his own.
The second is stranding a pronoun several sentences from its antecedent.
It was finally approved on Friday.
The third is using a pronoun where there is no antecedent at all.
In the report it says that costs rose. → The report says that costs rose.
All three are caught by one test. Read the sentence and ask how many words are competing for the role of antecedent.
Try it right now
Take a paragraph from your correspondence and mark every pronoun in it.
Draw an arrow from each to the word it refers to. If any arrow forks, that is where the noun has to come back.