The shadow function in Personality Type is not the same thing as the Shadow as a whole. Jung talks about the Shadow as everything in our unconscious that mirrors the conscious aspects of ourselves, and the qualities in the Shadow can be good or bad. If we are insecure, our unconscious Shadow tells us we are awesome. If we walk through life with confidence, our Shadow tells us we are worthless. Everything in our upper, conscious selves reflects the opposite quality in our subconscious Shadow.
In terms of Personality Type, the shadow function has a more specific meaning. It refers to the weakest of our personality functions—always the fourth, inferior function—that our personality struggles with. The shadow function is almost entirely unconscious, meaning that we have little conscious control over what it does, and it may drive us to think or act out in ways that go against our conscious moral code, or our general way of thinking and behaving.
As we've discussed elsewhere, each of the types (both Jungian and Myers-Briggs) uses four of eight possible personality functions, and rarely accesses the remaining four functions. You have a preference for a dominant, auxiliary and tertiary function, and these preferences define your personality type, and point toward the shadow function that will be in play for you.
Many of us know about the Anima/Animus, which Jung defined as our inner images of ideal femininity and masculinity. He believed these inner ideals are formed in our childhood, through exposure to the important people in our lives, and that as we grow up and begin to seek partners, we project our Anima/Animus onto people we become interested in sexually.
That's all fine and good, and perhaps anyone who's fallen in love can understand that feeling of luminous adoration for the object of your affection, and can appreciate that this feeling eventually fades. That's the Animus/Anima projection turning on and off, which is quite normal. In the case of long-term relationship commitments, most can and will fall into a pattern that holds friendship and partnership in high enough esteem that the couple can get past the loss of the initial burning of romantic love. In cases where the shadow function is involved, this is much trickier.
The shadow function, which invariably is very bad at whatever function it is shadowing, resents those who are good at it. If the shadow is Extraverted Sensing, it will cause feelings of resentment and dislike toward people with the Extraverted Sensing personality type. The Introverted Intuitive who is subject to this shadow function will dismiss Extraverted Sensers as "shallow" and "boring," and harbor a general dislike of them, except if there is an Anima/Animus projection in play, in which case the Introverted Intuitive will think the Extraverted Senser walks on water, right up until the projection ceases and they realize that they can't stand this person they used to adore.
I have no solutions for this problem—just mentioning. But it's a good idea to stay with someone who is very different from you a good long while before you decide to get married and make babies.
Introverted Intuition's shadow function is Extraverted Sensing. This personality's shadow operates in the concrete world outside of us, which can create a disconnect between the subject and a practical sense of reality. They will to some degree be unaware or unrealistic about the practical realities in the world around them, at least for the first half of their lives.. They might lose their keys a lot, miss their turns, or get into difficult situations by completely midjudging them, such as thinking they are going for a short hike and ending up slogging fourteen miles over a mountain.
Extraverted Intuition's shadow function is Introverted Sensing. This personality's shadow operates in the realm of subjective experience and memories, and for the first half of the Extraverted Intuitive's life, the shadow is likely to handicap their ability to take in their surroundings, and to warp the quality of their memories and past experiences. They will give themselves no credit for past achievements, but remember failures clearly, sometimes dwelling on them. They will look past the beauty of a sunset with disinterest.
Introverted Sensing's shadow function is Extraverted Intuition. This personality's shadow operates on its ability to see ideas and possibilities in the world around them, and for the first half of the Introverted Senser's life, this shadow is likely to cause them to dismiss anything that isn't an objective, easily-seen reality as far-fetched and impossible. It casts a dark and dangerous aura around everything that is not practical and immediate, causing the subject to be strictly pragmatic and unimaginative, and to be suspicious and distrusting of "hunches" or "gut instincts." In later years, the Senser might gain more trust in possibilities that aren't obvious, in which case the shadow will become a helpful aid to individuation.
Extraverted Sensing's shadow function is Introverted Intuition, which operates on the inner world of their imagination. For at least the first half of Extraverted Sensers' lives, they will not understand or have access to imagination or intuitive impressions, and will dismiss "hunches" as superstition. They will seek to live entirely in the world around them, valuing only that which is concrete and "real," shunning abstract concepts and discussions. Where others have intuitive impressions, the Extraverted Sensers have a blurry, dark void. Later in life, however far along the path toward individuation the Extraverted Senser has travelled will determine how effectively they can use their imagination.
Introverted Feeling's shadow function is Extraverted Thinking. This personality's shadow operates in the realm of concrete facts in the world around them, and will cause the subject to avoid and devalue using logic. They will be uncomfortable with Math and Computer Science, and might feel insecure about their ability to enter any conversation with an Extraverted Thinker, for fear they might be exposed for their lack of logical thinking skills. In extreme cases, where the Introverted Feeling function is very dominant in a personality, the subject will glom onto their ideals so strongly that everything else will be cast off as unimportant, even uncaring, or vaguely evil. The shadow will pump the subject with feelings of self-importance, while hating on the Extraverted Thinkers of the world.
Extraverted Feeling's shadow function is Introverted Thinking. This personality's shadow operates in the realm of inner facts and logic that are used to determine something interesting . An Introverted Thinking shadow will give the subject an aversion to deep, theoretical abstraction. They're likely to avoid conversations about philosophy or the meaning of life, or of anything that requires theoretical thinking, and will feel deeply insecure if forced to participate. This shadow can also cause the subject to pursue new theoretical projects in a chaotic way, with frenzied interest that soon dies out.
Introverted Thinking's shadow function is Extraverted Feeling. This personality's shadow operates in the world of social values, and as a shadow will cause the subject to be unaware and rejecting of what is accepted, what is revered, and what is unacceptable. They might be awkward, unaware of how to behave, sometimes making bad or inaccurate decisions about people and situations. For the first half of their lives, Introverted Thinkers will have this shadow influence that is like a toddler in their unconscious, insecure and trying out various ways to fit in. The shadow will push up thoughts that cause the subject to dislike and resent people with dominant Extraverted Feeling, who are so much better than they at understanding the feeling vibe, or empathy, of the culture around them.
Extraverted Thinking's shadow function in Introverted Feeling. This personality's shadow controls their understanding of subjectively-held value systems, and for the first half of the subject's life, will cause them to be unable to grasp or appreciate the concept of a personal belief system. They are likely to villify those who hold their own ideals above all else, seeing them as judgemental and aspiring for personal gain. Eventually, most Extraverted Thinkers who have close concourse with Introverted Feelers will begin to understand and see merit in personal beliefs, after which this shadow function can become a positive catalyst for individuation.