however可以单个使用，用作状语，表转折。例如:He said he helped me.However,he didn't.
However还可以引导让步状语从句，常放在句首。例如:However hard It is,I will finish it.
however和as 引导让步状语从句时的区别：
however引导让步状语从句时,相当于no matter how,后面跟形容词或副词；as引导让步状语从句时,用倒装语序,即把作状语的副词或作表语的形容词或名词提前,而且如果作表语的是单数名词,前面不加冠词,也可以把谓语动词部分的实义动词提前.
However adv. 无论如何；不管怎样（接副词或形容词）；然而；可是
conj. 无论以何种方式; 不管怎样
however没有而且的意思，它是连词，表示转折，“但是”的意思
however属于连词或副词
例句
1.
However, we do our best to keep the price as low as possible without sacrificing quality.
无论如何，在不牺牲产品质量的条件下，我们尽了最大努力，以尽可能低的价格向贵方报价。
2.
Chertoff cautioned, however, that serious security threats remain.
但是无论如何，切尔托夫也警告说，严重的安全威胁依然存在。
however是副词，表示然而，例如：
1.Many problems, however, escape detection.然而许多问题却未被察觉。
2.There was one respect, however, in which they differed.然而，他们在一点上有分歧。
3.He wanted to take no risks, however small.他多小的风险也不想冒。
however是副词，(与形容词或副词连用)无论到什么程度，不管多么；不管怎样；无论如何；然而；不过；仍然