Ok Денис, fine. Just be careful with, "helps us to close or open..."

Possible description of a KEY: A thin, metal object which is small enough to be held in the fingers. It has teeth of varying sizes and positions along at least one edge which form the code to unlocking, or locking, the door that it belongs to. Another type of key is the button for a letter or character on a computer keyboard. Similarly, the elongated buttons on a piano which each play a different musical note. A key can also be the important thing, (not always a tangible object), necessary to make something complete, so that it works or can be understood. So, the key to a door could be considered as the important thing needed to unlock it. The key to learning English is practice, practice, PRACTICE!

A key is a thing which helps us for closing or opening a lock.

Almost perfect, Kate. Nice and simple, again. Just be careful with "aN instrument". Sometimes, yes, the best way to describe something is by its function. Well done.

A key is a instrument to unlock doors.