Usage of 一 part 1

Character “一” (yī ) has a very simple shape. But it's usage is not simple at all.
First I want to tell you the story behind three characters. They are 一(yī )  二(èr ) 三(sān ).  The ancient Chinese people invented character 一(yī )  in the idea that one is the simplest element and also the whole cosmos. The massive cosmos was divided into sky and earth,  another character 二 (èr ), which means two, shows you the picture. In this character the top horizontal line refers to the sky and bottom line refers to the earth. Later on, human being showed up between the sky and earth,  then another character 三(sān ), which means three, is on the stage. In this character, the top and bottom lines refer to the sky and earth, and the line between means the human being. 

The basic meaning of "一"  is one. We need it when we count.

For example,

One person     一个人(yí ge rén )

One hour         一小时 (yì xiǎo shí )

"一" means "the first time, once"

一见钟情 (yí jiàn zhōng qíng )：fall in love at the first sight

一见 （yí jiàn ): see somebody or meet somebody for the first time
钟 ( zhōng ) :  Maybe you already know it has a meaning is 'clock' , but here it means ' focus, concentrate'
情 ( qíng ) :  feeling, passion

For example:
在一次聚会上，他俩一见钟情。
zài yí cì jù huì shàng ,tā liang yí jiàn zhōng qíng
At a party, they fell in love with each other at the first sight.

在…… 上   is an important sentence structure. The basic usage of this structure is to indicate that the position of an object is located at or on top of a surface.

Such as,
书在桌子上。
shū zài zhuō zi shàng .
The book is on the table.

But here 在…… 上 is used to describe "at a party, at a meeting , or at a ceremony " etc.
在一次会议上     at a meeting
在一次典礼上      at a ceremony, at a celebration

Related post:

Usage of 一 part 2
Tone Sandi of Character 一


If you check the following online dictionary, you will find more common words and idioms that have 一 as their part. 

参考象形字典