Because the Sun and Mercury are never more than 28 degrees apart, the only major aspect they can make in a natal chart is the conjunction.
Sun conjunct Mercury
Because the ego and the mind are aligned, people with Sun conjunct Mercury possess much mental energy. They are always in a position to think about what they want, and in many ways, this is an interruption of the will. Mercury brings definition to whatever it contacts, and when aligned with the Sun, the natives apply logic and definition to the will–and the will generally works better when it is allowed to “just be” rather than be analyzed. The result is much intelligence and communication ability, and a lot of pride invested in the intellectual capacities. These people may not listen as well as they speak, however! They might be too busy thinking about what to say next. As far as studying or learning goes, they are better off reading the material than listening to a teacher. These traits come from a strong need to take an active role in communications. It is very hard for these people to passively listen and absorb information.
Their opinions are usually strong and they are independent thinkers. They tend to be proud of their opinions and their thoughts, and might easily get a bruised ego if they are not “heard”, if their opinions are pushed aside or ignored, or if their opinions are criticized. Sun conjunct Mercury people are expressive and, in some cases, very animated speakers.
Some Famous People with Sun conjunct Mercury: Christina Aguilera, Ed Asner (Sun conjunct Mercury and Mars in Scorpio), Oliver Stone, Warren Beatty, Jeff Bridges, Woody Allen, Chris Rock, Barbra Streisand, Denzel Washington, Annie Lennox.
Sun parallel Mercury
This aspect in declination gives you a very active mind and quick wit. You relate well with others, and especially with younger people, although you can be quite nervous at times. You tend to retain a youthful quality throughout life. You tend to communicate with strength and you might captivate your “audience”. However, you may need to work on your listening skills if the Moon is not in harmony with the Sun or Mercury.