We’re absolutely certain that Tim Bowen can explain certain rules of word grammar.
The word certain is mainly used as an adjective but it can also function as a determiner and, more rarely, as a pronoun.
With the meaning of ‘having no doubts that something is true’, certain cannot be used before a noun, e.g. ‘I’m not absolutely certain, but I think I’m right’. However, with its other meaning of ‘definitely going to happen’, it can be used in the pre-nominal position, as in ‘Many of the refugees face certain death if they are sent back across the border’.
As a determiner, certain is used to refer to someone or something without being specific about who or what they are, as in ‘Certain basic rules must be obeyed’ or ‘There are certain things that we need to discuss urgently’. It can also be used in spoken English to refer to someone without saying their name when you know that the person you are talking to knows who you are referring to, as in ‘A certain person is going to be there, so you might prefer not to come’.
Used with the indefinite article, certain can mean ‘some but not very much’, as in ‘A certain amount of salt in your diet is good for you’. It can also be used to talk about a person when you know their name but nothing else about them, as in ‘The owner of the business, a certain Mr Knowles, was said to be living abroad’.
As a pronoun, certain can be used to replace some and is followed by the preposition of, as in ‘Certain of the criticisms made then and rejected at the time are now widely accepted’.