cannot; to be unable to; often used in set phrases like 不得而知 bù​dé'ér​zhī

his; her; its; their; refers back to a person or thing mentioned previously

door; gate; gateway; doorway; opening

Connects an adverb-like description to the following verb to show the manner, cause, or basis of an action.

to enter; to go into; to move from outside to inside