demand for ...表示“对 ...... 的要求”、“对 ...... 的需求”，这里的 demand 是名词。例如：There have been demands for their resignation. 要求他们辞职的声音持续不断。/ The demand for programmers exceeds the supply. 程式设计师的人才供不应求。/ Is there much [ a great ] demand for these goods ? 这些商品的需求量大吗？（摘自外研社、建宏《英汉多功能词典》）后两个句子中的 demand 是不可数名词。

ask for 是一个及物动词短语，主要有两种意思：（1）请求，恳求，要求 She asked my advice. = She asked me for advice. = She asked for my advice.她向我求教。/ She asked for a bicycle for her birthday. 她要求一辆自行车作为生日礼物。（2）要求（与人）见面，要求（人）来（接电话）A Mr. Simpson from Sydney is asking for the manager. 悉尼的一位辛普森先生要找经理谈话。