はい、僕は日本語を学んでいる。でも、まだよくない!
ぜんぜん文字しらない。
>>3を聞かない。かれはバカだ。
The English in your post >>5 is excellent.
>>I would answer as far as I know.
Here, it would sound a bit better to say "I would answer as best I can."
>>I've read English sentences only in a textbook
Here, it is more correct to put 'only' at the beginning, like this:
'I've only read English sentences in a textbook.'
Sorry, I can't really explain the rules about these things, because I don't know them myself. Perhaps other posters can comment.