Your teacher is correct.
example
"Juan fue a la caja y cogió el único anillo de entre las joyas"
Juan went to the vault and took the only ring among the jewelery.
"Juan fue a la caja y cogió el anillo único de entre las joyas"
Juan went to the vault and took the special ring among the jewelery
But it's more easy to understand if you know "Lord of the rings" or "the Hobbit" by J. R.R Tolkien (a must-read)
Introduction
Not far away was his island, of which Bilbo knew nothing,
and there in his hiding-place he kept a few wretched oddments,
and one very beautiful thing, very beautiful, very wonderful.
Gollum had a ring, a golden ring, a precious ring.
"My birthday-present!" he whispered to himself, as he had
often done in the endless dark days.
"That's what he wants now, yes; we wants it!"
He wanted it because it was a ring of power, and if you slipped that ring
on your finger, you were invisible; only in the full sunlight could you be seen,
and then only by your shadow, and that would be shaky and faint.
"My birthday-present! It came to me on my birthday, my precious."
end of introduction
So it was a vey special ring. But also was the only one ring there.
He had a ring, a golden ring, a precious ring.
Tenía un anillo, un anillo dorado, un anillo precioso.
Tenía un único anillo, un anillo dorado, un anillo precioso.
He had only a ring, a golden ring, a precious ring.
It means that in the place was only a ring. This ring was golden. This ring was precious.
It becomes more important with the reading. But its only a precious golden ring, wich is alone.
Perhaps you can find it somewhere else. (worth 1000 $??)
Tenía un anillo único, un anillo dorado, un anillo precioso.
He had a unique ring, a golden ring, a precious ring.
It was the ONE ring. It was golden. It was precious
The ring is one in the world. It was golden. It was precious.
You can't find one like it. It's also golden and precious (worth the Middle-earth ??)
Hope it helps for "único"
For differences in adjectives, ask Tomissimo
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http://en.wikipedia.org/wiki/Lord_of_The_Rings
This is the Master-Ring, the One Ring to rule them all.
This is the One Ring lost many years ago,
to the great weakening of its maker's power.