She forgot which things_to buy
you should force your attention on your study.
the teacher asked Tom to pay more attention to his handwriting
第一个,which things to buy是疑问代词which things加不定代词to buy（do）,这是固定结构,必须记住.第二个和第三个是pay attention to do,force one' s attention 这是一个固定短语,要记住,多背短语,都记句子,多背课文,会有好处的,希望你可以借鉴,