Competition for these jobs is very severe—we have five times as many applicants as we did last year. 

中文原句：谋求这些职位的竞争很激烈，今年的求职者是去年的五倍。

1. 此句competition 前面为何不需要定冠词 the？

2. Competition for： 介词 for 表示“对于，目的”是吗？请老师讲一下 for 和 of 的精确选用？

一、competition 主要有两种意思：“竞争”和“比赛”。

（一）表示“竞争”，是抽象意义，不用不定冠词，没有复数，可以有some，much等词语修饰。即便有定语修饰（如：Competition for the job），也很少加定冠词特指。例如：

1. The film faces stiff competition for the Best Film nomination.

这部电影在角逐最佳影片提名时面临着激烈的竞争。

来自柯林斯例句

2. Competition for the job is very severe.

这份工作的竞争是很激烈的.

来自《简明英汉词典》

3. There's been some fierce competition for the title. 

夺冠之争一直都相当激烈。

来自辞典例句

4. Reduction of competition for food stimulates both natality and survival. 

食物竞争的减少,刺激了出生率和存活力。

来自辞典例句

（二）表示“比赛”，是具体意义，它是普通名词，使用时需要加冠词，有单复数。例如：

1. Will you judge at the swimming competition next week?

你愿意担任下周游泳比赛的评判吗?

《21世纪大英汉词典》

2. She went in for a singing competition.

她参加了一项歌唱比赛。

《21世纪大英汉词典》

3. But in every competition there can only be one winner.

但在每一个比赛中只能有一位优胜者。

4. He participated in many sports competitions.

他参加了许多场体育比赛。

来自《权威词典》

二、for 表示“目的”和“针对”意义；of 表示“所有关系、所属关系”，用在此处显然不合适。

of 表示“所属关系”，跟表示目的的for，一般不会混淆，最容易混淆的是介词to。我倒觉得，of 跟表示“针对关系、对象关系”的to，容易混淆。例如，以下表达用to，不用of：

the answer to the question

the solution to the problem

the response to the inquiry

the reaction to the proposal

the reply to the letter

the key to the door

notes to the text

the exit to the cinema

the entrance to the hall

the bridge to knowledge

the way to school

the road to victory

the key to success

a visit to the Great Wall

a letter to the president

the Monument to the People's Heroes

a challenge to the theory