$\textit{My answer :}$

(3) - mistake : $\text{"a"}$ 

      - explanation : because " real danger " is specific so we can't use "a" 

      - correction : but instead, we should use $\text{" the  "}$

(4) - mistake : $\text{"city"}$

      - explanation : we have to use the  plural-noun because " their "

     - correction : $\text{" cities "}$

(6) - mistake : $\text{" harmonious"}$ 

  - explanation : we should add more relatives word for the explanation 

      - correction : $\text{" that is harmonious "}$

(8) - mistake : $\text{" style in "}$

      - explanation : grammar in use 

      - correction $ \left[ \begin{array}{l}\text{" style of" }\\\text{" life style" } \end{array} \right.$