1. before还是ago?

要说：

I came to Manchester two weeks ago.I came to Manchester two weeks ago.

不要说：

I came to Manchester before two weeks.

Use "past time + ago"

ago用于一般过去时，例如：

10 years ago, a long time ago, a few weeks ago, a couple of days ago

Use "before + action / event"

before用于动作或事件前，例如：

before you left, before we start, before lunch

2. fall 还是reduce?

These two words are the opposites of 'rise' and 'raise':

它们是rise和raise的反义词：

something falls (e.g. the price fell)

somebody reduces something (e.g. the company reduced the price)

You will probably use 'fall' to describe numbers on a graph or chart for IELTS Writing Task 1:

在雅思短作文中，你可以用fall来描述图表里的数字。

In 2008, the number of customers fell. (verb, past)

这里的fall是动词，过去时。

There was a sharp fall in Internet usage. (noun)

这里的fall是名词。

'Reduce' and 'reduction' are probably more useful for Writing Task 2:

reduce和reduction在长作文中更有用：

We need to reduce the amount of pollution from exhaust fumes. (verb)

reduce，动词。

Recycling leads to a reduction in waste. (noun)

reduction，名词。

3. because还是because of?

Compare these two sentences:

比较下列两个句子：

I stayed at home because the weather was bad.

I stayed at home because of the bad weather.

The important difference is the use of the word 'was' in the first sentence. After 'because', you need a subject and a verb (e.g. the weather was). After 'because of', we don't use a verb, only a noun, nouns or noun phrase.

重要的区别在于第一句里使用了word。Because后面需要加主语和动词(例如the weather was)。Because of 后面不需要动词，只要一个或多个名词，又或者名词词组就可以了。