`16.` She went to bed because she was tired.
`⇒` Because + S + V + (O).
`17.` If you don't study hard, you'll fail the exam.
`⇒` If + S + V s(es), S + will / can/ may (not) + V (infinitive)
`18.` I suggest that you should work harder on your pronunciation.
`⇒` S + Suggest + that + S + V ...
`19.` Our environment is extremely polluted so we should prevent people from littering.
`⇒` S + V, + So + S + V
`20.` Shall we go to the beach now?
`⇒` Shall + S + V (infinitive) ...

Chúc bạn học tốt! Nếu có thắc mắc gì bảo mình nha. ^^

`16 - 21`.She went to bed early because she was tired

`\rightarrow` Because + C1 + C2 : Bởi vì

`17 - 22`.If you don't study hard , you'll fail the exam

`\rightarrow` CĐK Loại 1 :

`-` If - clause : Thì HTĐ : S + Vs / es / không s , es

`-` Main - clause : Thì TLĐ : S + will + V

`18 - 23`.I suggest that you should work hard on your pronounciation 

`\rightarrow` S + suggest + that + S2 + should + V : Gợi ý nên làm gì

`19 - 24`.Our environment is extremely polluted , so we should prevent people from littering

`\rightarrow` So dùng để chỉ kết quả

`20`.Shall we go to the beach now ?

`\rightarrow` Shall + S + V ? : Nên làm gì

`\text{@ TheFox}`