Incorrect spelling


Incorrect spelling, explanation: the word writer comes from the verb write with suffix -er. Therefore writter doesn’t make sense as t is in the middle of the core verb and there is no need to double it. This means writer is the correct form.

Correct spelling


Correct spelling, explanation: this form is the correct one, because it is a noun created from the verb write, and by adding the suffix er it gets a meaning of a person, who does the particular action. Therefore writer is correct – it’s simple suffix -er (in this case r, since the verb ends with e) added to the verb. Writter is incorrect because t sound is in the middle of the core word; there is no point in doubling it. Writer is therefore the correct form.

Definition of writer:
noun, someone who writes, usually for a living; an author of a piece of writing
My favourite writer is Haruki Murakami.
Phil used to be a novelist but then he became a screenplay writer.