比如说please have -------look at the photo on the wall 要不要用a或an?

需要a
have a look at .... 是固定搭配

用a

have a look at是“看”的意思，表示看什么具体的东西，at后面要接宾语。

用a
have a look是一个词组

have a look 是一个固定短语,此处的look 是名词
have a look at sth.
要用 a 的