一个关于不定冠词用法的问题想问一下不定冠词通常什么时候用 为什么洗澡用have a bath,而吃午

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1 第一次出现的名词要用不定冠词
2 一些特殊短语（需要加强记忆的）不需要问 为什么比如你举的例子.
3 名词音标中以元音开头的要用an 比如an egg
======以下答案可供参考======
供参考答案1:
因为bath可用做动词，而lunch是绝对的名词
供参考答案2:
定冠词的用法
(1) 由于上下文的关系, 某名词所指的东西已非常明显, 或已有一定的范围, 该名词应加the。
Shut the door.
Please pass me the caster.
(2) 表该名词的总称。
The cow is a useful animal. (= Cows are useful animals.)
(3) 前面已提过的名词，再度提到时前面加the。
Once upon a time there was a little boy who lived in a cottage.
The cottage was in the country, and the boy lived there all his life.
(4) 用于为片语或子句所修饰的名词之前。
This is the book that I promised to lend you.
(5) 用在宇宙（独一无二）的天体名词之前。
The sun; the moon; the sky; the earth.
(6) 用在方位，方向等名词之前。
The sun rises in the east and set in the west.
(7) 乐器的名称前要加the。
To play the piano (violin, guitar, flute, etc.)
(8) 表示单位的名词前要加the。
Gasoline is sold by the gallon.
Meat is sold by the catty.
(9) 在最高级的形容词或序数(the first...)之前要加the。
Taiwan is the best place to live in.
(10)专有名词在下列情况可加the。
(A) 海、河、运河的名称：The Thames; the Bahamas;
(B) 复数形的专有名词: The West Indies; the Bahamas;
(C) 船泊的名称: The Queen Mary; the Flying Scotsman; the Comet;
(D) 书报、杂志、经典的名称： the Times; the Central Daily News; the Koran;
(E) 公共建筑、机关的名称: The Ritz Hotel; the Ministry of Education;
(F) 国民的总称: the French; the Japanese; the Chinese;
(G) 用在姓氏的复数前指其全家人: The Chens will move to the country.