小题1:It’s dangerous to leave a child/children alone at home.

小题2:The dancer is always worrying about getting fat.

小题3:Telling lies can make things worse.

小题4:When we have difficulties with our study, Mr. Wu is always ready to help us.

小题5:I don’t think teenagers should be allowed to stay out late.

题目分析：

小题1:It’s+形容词(+for sb.)+to do sth.做某事对某人来说…..;it做形式主语，真正主语是不定式。

小题2:worry about担心，后接动词的-ing形式。

小题3:tell lies 说谎，用动名词形式做主语；worse更糟糕，bad比较级。

小题4:have difficulties with sth.,对……有困难；be ready to准备做……。

小题5:句子用到被动语态，be allowed to do允许做某事。