many girls have long hair,为什么用have
解析：many girls是句子的主语，复数主语谓语动词用复数形式（即原型），所以用have是正确的。
关键词汇：have
音标：英[hæv , həv] 美[hæv , həv]
详细释义：
vt. 有; 持有; 占有; 由…组成; 显示出，带有(性质、特征); （通过行动）表现出（品质）; （表示关系）有; 能用; 有责任（或义务）; 须要，有必要（做某事）; 抓住; 使放在; 感到; 患病; 经受; 组织; 吃;
aux. 与过去分词连用构成完成时;
n. 有钱人; (天然资源多的)富国; 诈骗;
短语搭配：
to have sth from sb 从某人处得到某物
to be having a baby 怀孕
to have a meeting 开会
to have an idea 有主意
to have sb to stay 留某人过夜
例句：
I've been had by that swindler.
我被那个骗子骗了。Such books may be had at any bookstore.
这种书可以在任何书店里买得到。She'd been had before she got married.
她在结婚前已和别人发生过关系。
词形变化：
动词过去式: had
动词过去分词: had
动词现在分词: having
动词第三人称单数: has
因为这里表示 有；持有；占有 的含义。（作某些意义时也用 have got，尤其是英式英语。）
例如：
He had a new car and a boat.
他有一辆新车和一条船。
She's got a BA in English.
她有英语学士学位。
have的其他含义:
由…组成
In 2008 the party had 10 000 members.
这个党在 2008 年时拥有 1 万名党员。
显示出，带有（性质、特征）
The ham had a smoky flavour.
这火腿散发着一种烟熏的香味。
（通过行动）表现出（品质）~ sth to do sth
Surely she didn't have the nerve to say that to him?
她一定没有胆量跟他这样说吧？
能用
Have you got time to call him?
你有时间给他打电话吗？
有责任（或义务）；有必要（做某事）
We have a duty to care for the refugees.
我们有义务关怀这些难民。
I must go—I have a bus to catch.
我必须走了，我得去赶公共汽车。
抓住
He had his head in his hands.
他双手抱着脑袋。
使放在；使保持（在）~ sth + adv./prep.
I soon had the fish in a net.
我不一会儿就网住了那条鱼。
感到；想到
He had the strong impression that someone was watching him.
他强烈地感觉到有人在监视他。
患病；得病；染病
I've got a headache.
我头痛。
经受；经历；经验
I went to a few parties and had a good time.
我参加了几次聚会，过得很愉快。
组织；举办
Let's have a party.
我们办一次聚会吧。
吃；喝；吸（烟等）
to have breakfast/lunch/dinner
吃早饭／午饭／正餐
进行（活动）
I had a swim to cool down.
我游了泳，凉快凉快。
