英语：为什么不能说these work,只能说these tasks

I 'm reading a new book these days,one in English.为什么不能说it's 如过对方说:I can not concentrate on my work these days.我该说什么啊(用英语 为什么不能说what's your work? 只能说quite sure ,为什么不能说very sure? These shoes need()at once为什么只能填mending不能mend 即二者区别? 这里只能说英语?不能说中文?英文! 英语翻译these tasks are performed primarily through advertising- 为什么说She denied having seen these necklaces 而不说She denied has "These glasses?"可否说成This one或This glasses"为什么? 为什么只能说will be more pollution而不能说will have more pollution 为什么不能说My hometown has changed a lot.只能说Great changes have ta 为什么只能说重力与质量成正比,不能说质量与重力成正比?