$\textit{My answer :}$
(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.$