就当没看见用英文怎么写

Leave it alone.(比较正宗口语话表达，意思是别管它，就这样吧，符合你说的“就当没看见”的意思）
硬要按字面翻得话就是“Take it as you've never seen it.'个人感觉这样较生硬，不太好

Should not have seen right away

O(∩_∩)O~
自己不是很会，在百度翻译上查的..

就当没看见：just pretend not to have seen it