Incorrect spelling, explanation: we do not double t in writer because t isn’t the last letter in the core word write. Therefore we have no reason to think about doubling this consonant, since e is the last letter in the word we are adding a suffix, so writter is incorrect. The correct form is writer = write + -er.

Correct spelling, explanation: writer is the correct form of this noun. This is due to the fact, that it is made from the verb write, there is no reason to double consonant t (as it’s not the last letter in the word). In this case suffix -er was added; because of e being the last letter in write, it was skipped in the suffix, hence writer.

Definition of writer:
noun, a person who writes, usually a book author
In my opinion, Lew Tolstoy was the greatest writer of all time.
Are you a writer? Cause you have a talent!

Phrases with writer:
writer’s block
He hasn’t written anything for years; he suffers from writer’s block.