What's wrong with Nikon d7000 shooting a short film for only 6 or 7 seconds?

You bought a low speed card. The card you gave you when you bought it was not the original card, but a cheap low-speed card that the dealer bought casually. It can shoot for 20 minutes, which is the total shooting time. You can lower the resolution, say 640*480 (I'm not sure about this), and maybe you can record it for a longer time. However, if you want to record longer, changing cards is the first choice.

You can choose a Class4 card, that is, the writing speed is at least 4MB/S (the reading speed is generally 20M/S, so don't confuse the writing speed with the reading speed). I used Panasonic's 2G SD card (class4) to record video on 550D, and the cards were all 2G cards, with no cards and no stops.

Class6 card is better in principle, but on the one hand, it is not easy to buy, on the other hand, the price is on the high side ... level 4 is enough.