一个关于不定冠词用法的问题

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

因为bath可用做动词，而lunch是绝对的名词

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