we couldn't go for a walk because of the heavy rain

`=>` because it rained heavily,we couldn't go for a walk 

`-` because +mệnh đề, mệnh đề.

 = because of +  cụm danh từ / V-ing, mệnh đề

- cụm danh từ : adj +N

Đáp án: Because it rained heavily, we couldn't go for a walk.

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Because + Clause, clause 

= because of + N / NP / Ving, Clause

--> bởi vì 

S + V + ADV 

S + V2/ed (thì QKĐ)

`color{orange}{~LeAiFuRuiHuiBao~}`