certain

1 certain /ˈsɚtn̩/ adjective
1 certain
/ˈsɚtn̩/
adjective
Britannica Dictionary definition of CERTAIN
1 
not used before a noun, [more certain; most certain] : not having any doubt about something : convinced or sure
2 
— used with it to say that something is known to be true or correct
3 
— used to say that something will definitely happen or that someone will definitely do something
— often followed by to + verb
4 
always used before a noun — used to refer to something or someone that is not named specifically

a certain

1 
— used to refer to a quality that is noticed but that is difficult to explain or describe
2 
formal — used with the name of a person you do not know

for certain

: without a doubt : definitely or certainly

make certain

: to do something or find out something so that you have no doubt about whether something is true, correct, will happen, etc.
— sometimes + of
2 certain /ˈsɚtn̩/ pronoun
2 certain
/ˈsɚtn̩/
pronoun
Britannica Dictionary definition of CERTAIN
formal
: certain ones : particular members of a group : some — + of