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Once I was witnessing with a group of Christians at a bus stop. One man we met was Christian, but seemed to be quite bitter and angry about life. I discerned he was struggling with the sin of greed which was putting a stranglehold on his life, while others in the group judged he must not really be a Christian because his demeanor wasn't 'nice'.
That is a primary difference between discernment and judgement. The Holy Spirit helps us discern good from evil, or discern the spirit behind something. Judging is passing a sentence on something. This can be good, such as judging that adultery is wrong because God said so, or discerning that a teaching is false so making the judgement that you should not listen to it. However, it can be bad if used to judge a person, such as judging them to be not a Christian, or judging them outside of the reach of God, or judging them to be a worse sinner than oneself, etc.
Judgment a final decision based off of assumptions or what you think of something; discernment is understanding, connecting dots.