A subtle difference is so slight that it is difficult to detect or describe.
EXAMPLE: The two shades of green were so subtle, that you needed to put them next to each other to tell that they were not the same color.
A simple sentence uses a subject and a verb..
To avoid being obtrusive in the elegant restaurant, he used subtle gestures to call the waiter to his table.
She chose fabrics in subtle shades of grey.
The dawn revealed subtle nuances in the coloration of the rocks. That hint was not very subtle.
Not many people noticed the subtle humour in his remarks.
Some spices have a subtle aroma. The wink she gave was no subtle clue.
Complex (APEX)
Like this; I am an architect.
i went on an expedition
I am unsympathetic to your need.
i am a dissident
The field had boundaries.