No difference between THE, AN or A ?Yes, there is a...

No difference between THE, AN or A ?
Yes, there is a difference.
Only you have full legal ownership of your car, are you AN owner or THE owner of that vehicle ?
In this case, the owner is correct.
An owner is correct if there is more than one owner.
That is the English language for you.
The apostle preaching to X means the only apostle preaching to X. An apostle preaching to X means he/she could be one of a few or many.
It doesn't necessarily mean that what is being preached is unique.
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Z was the maths teacher at my high school, which means that Z was the only maths at my high school, but he taught the standard high school maths at the time.