That form is informational only ... to be used when you sell the underlying stock to determine basis.
So suppose I purchased vested stock then I do not have to fill in any information about that and it is not tax deductible ?
Stock purchases are not tax deductible nor are they taxable until you sell them.
why then after I import my Form 3921, the tax I owe increases dramatically ?
You cannot import an informational form ... the sale of stock should be on a 1099-B and you use the info from the 3921 to report any missing info from the broker if needed. <a rel="nofollow" target="_blank" href="https://www.irs.gov/pub/irs-pdf/f3921.pdf">https://www.irs.gov/pub/irs-pdf/f3921.pdf</a>