In the title, the selling price is 2040, the closing price is 20 10, and the profit is 30 yuan.
If the spot price of soybean drops to 1990 yuan at this time, then the spot loss is 30 yuan, and the futures profit just makes up for the spot loss. At this time, the basis is still -20 yuan. Only when the spot price is higher than 1990 yuan, the futures profit will make up for the spot loss and there will be additional profits. For example, if the spot price is 2000, there will be a profit of 10 yuan/ton, and the basis difference at this time is 2000-20 10=- 10 yuan, which is greater than -20 yuan.
There are some concrete conclusions in the qualification textbook. You can learn about them first and remember them more easily in the future.