Some more info on thread breaking:
80% of the time, when the thread keeps breaking, it is because the needle has a rough spot or it is in backwards. If it is in forwards, replace the needle. It can also be a burr some place.
15% of the time it's because someone left the thread on a windowsill and the light "ate" the thread until it was weak. If your thread has rotted, roll off two whole outside layers of the spool and try again. If the spool has been in sunlight too long, you may need a new spool of thread altogether.
5% of the time thread breaks because the "timing" of the machine is off. Industrials usually include retiming instructions in the manual of operation. Home machines usually require a trip to the shop to fix this.