the past tense and the perfective?

Question

The weather in Taiwan_____a lot in the past few years because of the serious air problem year after year.

(A)has changed (B)changed

The answer is (A), but why can't I fill (B) in? Isn't the tense "in the past few years"?

Some told me that both the past tense and the perfective can be used to show the past time, yet the perfective is used in "a indefinite past time" and the past tense is used in "a definite past time point" like the two below:

I have written the letter several weeks ago.
I wrote the letter two weeks ago.

Is what my friend told me right? Please help with the two tenses I need help to deal with my exam but I'm a Eng. learner so just help me please.

Answer 1

A. has changed. I HAD written the letter... or, I wrote the letter.... Depends on the context. 'I have written' is wrong no matter how you use it. It would never be right with 'several weeks ago'. The only way it would be correct is to say, 'I have written the letter.' In fact, I wrote it several weeks ago.

What has the weather in Taiwan done? It has changed.

What did the weather in Taiwan do? It changed.

While either answer satisfies the questions, 'it changed' means that at one time the weather was one way, and now it is different. 'Has changed' means that the weather gradually went from one condition to another, and may change more in the future. 'It changed' implies that it will never be the same as it was in the past.

'Did' means that something occurred at one time and is not on-going. In the Fall, you would say the leaves have changed color. Any other time of year, you'd say that the leaves did change color. In the Fall, the color change is still happening; any other time of year, the color change is something that has happened and is over with. Did means done. Has means still happening.

Really hard to explain. Thankfully, I was taught to speak correctly from childhood; I don't need to know the rules. I know right from wrong intuitively.

Answer 2

This website might help you with your problem:

http://www.english-the-easy-way.com/index.htm

You can find others by googling "use of past and perfect tenses in English."