Hello Zhengya, Using 'were' in your sentence is a conditional tense,where you use it to refer to unreal Conditional and imaginary situations. In general auxiliary verbs are used both in oral and written English with no discrimination. Auxiliary verbs ( also called helping verbs) are used with another main verb. They can't be used with modal verbs such as can, will. You can't say for example: " I am can . " "I do will ." etc Auxiliary could be used to make sentences negative or used to ask questions. The 3 most common auxiliary verbs are : Do , Have , Be Read more about auxiliary verbs on the following: * http://grammar.ccc.commnet.edu/GRAMMAR/auxiliary.htm * www.answers.com/topic/auxiliary-verb

Hi Zhengya, for more grammar tips, have a look at this Italki site http://www.italki.com/knowledge/contribution-ENGLISH+GRAMMAR.htm

这是虚拟语气的内容，用If I were you……，表示现在的情况和事实相反，不可能发生的一个假设，假设如果我是你……（但是我是你的这种情况永远不可能发生）。注意：If I were you……，If he were you……，,正常情况下第一、第三人称用was,但这里统统都用were,因为这里用的是虚拟语气。关于虚拟语气和情态动词的使用不是在这里一两句话能说的清楚的，我只是在这里做个抛砖引玉。要找本语法书好好看看。

"If" makes the tense conditional. "were" is the conditional tense of "to be". Notice that "would" is also the conditional form ("I'd" = "I would") In normal spoken English, you might hear the incorrect form: "If I was you..." This uses the past tense instead of present. It is strictly incorrect, but you will hear it a lot. English has too many tenses doesn't it? One more thing: "is it used often in oral English?" or "is it used very much in oral English?"