我个人觉得还是语法问题啊....

Sorry,I type English much faster than I do Chinese.

From a grammar stand point, "Bear*****ack" has given you very good answers already. I personally want to stress "grammar" still. Because, if you firmly believe that adjectives cannot modify verbs, then "Don't take it serious" is simply wrong.
On "I'll study tonight. seriously" vs. "I'll study tonight. serious.", both are correct - refer to Bear*****ack's answer.

The bottom line is grammar. A solid command of grammar is extraordinarily important to us foreigners because we adults rely on analytical learning of a foreign language.

Hope above is helpful. Thanks.