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,
They had a congenial talk.
Her eyes were red with crying.
This child is extremely intelligent.
In some cases, the complement following DE (得) is a mixture of the above-mentioned types：
She is so busy that her life is a mess.