The main function of DE(得) is a complement marker which follows either a verb or an adjective. It is normally used to introduce the complement of manner, result and degree of an action. It usually occurs in a narrative sentence which is already happened. For example,
他們談得很投機。(manner)
They had a congenial talk.
她哭得眼都紅了。(result)
Her eyes were red with crying.
這個孩子聰明得很。(degree)
This child is extremely intelligent.
In some cases, the complement following DE (得) is a mixture of the above-mentioned types：
她忙得一蹋糊塗。(manner+result+degree)
She is so busy that her life is a mess.