Certain has 12 different meanings across 4 categories:
Adjective · Det · Pron · Proper Noun
Something certain.
"After waiting all day for a response, I finally felt certain that he had forgotten to send me the files."
In plain English: In this context, certain is not a noun but an adjective used to describe something that is known for sure without any doubt.
"The certain was never mentioned in her will."
Usage: Use "certain" only as an adjective or adverb; it has no standalone meaning as a noun referring to "something certain." Instead, refer to that concept by using the phrase "a certain thing" or simply "something."
definite but not specified or identified
"set aside a certain sum each week"
"to a certain degree"
"certain breeds do not make good pets"
"certain members have not paid their dues"
"a certain popular teacher"
"a certain Mrs. Jones"
established beyond doubt or question; definitely known
"what is certain is that every effect must have a cause"
"it is certain that they were on the bus"
"his fate is certain"
"the date for the invasion is certain"
Sure, positive, not doubting.
"I am certain that we will win the game today."
In plain English: Certain means being sure or completely confident that something is true or will happen.
"I am certain that we will arrive on time."
Usage: Use certain when you are absolutely sure something is true or will happen, often contrasting with uncertainty. It functions as an adjective to describe facts you hold without doubt, such as in the phrase "I am certain it will rain."
Having been determined but not specified. The quality of some particular subject or object which is known by the speaker to have been specifically singled out among similar entities of its class.
"The certain patient in that ward has finally shown signs of improvement after weeks of treatment."
Unnamed or undescribed members (of).
"I need to ask certain people at the office about the new policy before we finalize our plans."
A surname.
"The editor confirmed that Mrs. Certain was indeed a renowned local historian before inviting her to speak at the town hall."
The word "certain" entered English from the Old French certain, which derived from a Vulgar Latin form meaning "fixed or resolved." It replaced native English words like wis and iwis that previously held this meaning.