Whenever you sell any asset, you owe capital gains tax if there is a gain. If you owned the asset 1 year or less, the gain is taxed as ordinary income (generally 22% or higher). If the asset was held more than one year, the gain is taxed as a long term gain at a lower interest rate (generally, 15%).
Special to your personal home, if you own your home and occupy it as your main residence for 2 years, you can exclude the first $250,000 of gain from your income, meaning it is not taxed at all (or $500,000 if married filing a joint return). The tax consequences of selling your home early are that you pay normal capital gains tax and don't get the benefit of the exclusion.
However, if you are selling due to a change of circumstance that creates a hardship to stay in the home, and the change of circumstance was unforeseeable at the time you bought the home, you may qualify for a partial exclusion. This is described on page 6 of publication 523.
https://www.irs.gov/pub/irs-pdf/p523.pdf
There are some "safe harbors" (circumstance that qualify), and also a general category of "hardship". For example, if you sell in September after owning the home for 8 months, and you claim a qualifying hardship, then you get 8/24th of the usual exclusion ($83,333 for single or $166,666 for married filing jointly) and you pay short term capital gains tax on any gain over that exclusion limit. Please review all of publication 523, then feel free to reply with any clarifying questions.
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