I know that, for example Si, is an insulator, but when you add impurities it becomes a n-type or p-type semiconductor. And a semiconductor can only conduct electricity under certain conditions, but why does a semiconductor only conduct electricity under certain conditions? And how does that make semiconductors useful for many electronic gadgets if there is a chance it will stop conducting electricity, causing the gadget to stop working?

Semiconductors have an electrical conductivity between that of a conductor (like copper or aluminum) and an insulator (like rubber or glass). This unique nature between conducting and non conducting makes them to work as per desire in electronic gadgets. Engineers can control these conducting and non conducting tendency through the application of voltages or other external factors and can design circuits that perform specific functions reliably.
There is always a risk of semiconductor failure and modern manufacturing techniques and modern materials are refining and extending the life of electronic devices working on semiconductors.

Ok, thank you!