This is the 789th most frequent Japanese word.
書か
"書か" is a conjugated form of the verb "書く" (to write) in Japanese.
Here, "書かない" is the negative form of "書く," meaning "to write." It shows the action of writing and its negation.
私は日記に毎日書かない。
I don't write in my diary every day.
The term "書かれている" is the passive form of "書く," conveying that something is written by someone.
彼は説明書に何か書かれていると言った。
He said something is written in the manual.
In "書かないと," the expression indicates a conditional where not writing leads to a specific consequence.