2.Water helps keep your system clean.

水有助于保持人体内部系统清洁。 "keep＋宾语＋宾语补足语"结构，意为"使......怎么样"。 This woolen sweater may keep you warm in winter.

这件羊毛衫在冬天可以保暖。 3.Experts suggest that teenagers spend at least 30 minutes exercising，five times a week.

专家建议，青少年每周至少应该锻炼五次，每次至少30分钟。 suggest意为"建议"，宾语从句用虚拟语气。 They suggested that the factory (should)make full use of the equipment.

他们建议工厂充分利用这些设备。 4.That is easy to do！

那一点很容易做到！ 主语＋be＋adj.(easy，difficult，hard...)＋to do sth.不定式用主动形式表示被动含义。 The book is easy to read.

这本书很容易看懂。 5.Walking and riding your bike count，and so do school sports.

步行和骑自行车就算，校内体育活动也算。 "so＋系动词/助动词/情态动词＋主语"表示前面的肯定情况也适用于后者。 I have been busy today，and so have you.

我今天一直在忙，你也是这样。

　　 (教材P58)Diets are useless in the long term，yet approximately 20% of teenagers say they have tried going on a diet and skipping meals to control their weight.

　　从长远角度来看，节食并不能奏效，然而有大约20%的青少年声称，他们曾尝试过节食或者有时不吃饭，以控制自己的体重。

(1)useless adj.无用的，无效的