The 12th house is a closed house, a house of solitude, immersion in oneself, in one's inner world. Accordingly, a person with such a position of Mercury will not openly express their thoughts. But Mercury is a living planet, it needs food for the mind. Yes, such a person likes to learn, but he won't demonstrate it.
It is in solitude with himself that the person is comfortable thinking about something. It is in solitude that he forms the final thought forms in his head. And more often than not, such people tend towards distance learning.
Such a person has a great interest in occult knowledge, in some secret information. And he may have secretive work, which does not mean that he works in a closed organization, but he may have access to confidential information, to secret documentation.
A child with such Mercury, especially if he is not quick in speed of movement along the celestial ecliptic, may have difficulty in studies. He may not be able to catch up with the teacher's knowledge, not because he is stupid, but because such a child needs comprehension, digging into the material.
Such a person may be taciturn. But he can write and communicate his thoughts in writing very well.