Consider:
This is the only useful book available here.
I am the only child of my parents.
No other one can be found on the island. The hermit is the sole inhabitant there.
This widow lived all by herself. Her sole companion was her maid servant.
The two women were the sole survivors from the air crash.
Phil is the sole heir to all that property.
Are these two words interchangeable in the examples above?
Best Answer
"Sole" means "the only one". "Only" means the set is restricted to the identified members, but there could be more than one in that set.
That is, in all your examples, you could use either "sole" or "only" and the sentence would mean the same thing, EXCEPT for #5. You cannot say, "The two women were the sole survivors" because "sole" means only one. Effectively you're saying, "Only one person survived: Sally and Jane", which makes no sense.
That said, we commonly say "He was the sole survivor ..." or "the sole heir". But I've never heard someone say, "It was the sole useful book ..." or "I am the sole child ..." I'm hard pressed to think of a rule for when one is used versus the other in cases where either is valid.