grammar and tense problems

Hi,
2) I saw the boy singing. <------- Correct
I saw the boy singing.= I saw a part of his action( singing)
I saw him singing<-----------He was holding a mic or something.
Usually we say " I heard 聽到 the boy singing.".
3.I saw the boy sing.<----------Correct
I saw the boy sing. = I saw the boy sing (from the beginning to the end)
1) I saw the boy was singing./I saw that the boy was singing.
( saw) here may mean" to understand".
I think you want to talk about the action you saw right?
Use 2 or 3 then.
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And I would like to ask the difference between past tense and present perfect tense.Please explain in datail.
"past tense" is used to talk about things in the past.
My mother died three years AGO.
" three years ago" here refers to the past.
He was singing. ( past progressive tense.) WAS singing( on-going action)
He had 已經left when I arrived. had left
To show which event took place first.<------------
http://www.englishclub.com/grammar/verb-tenses_past.htm
Yahoo only allows me to type 300 words here, so
I cannot show you all. Please go to the website above and learn more.






The present perfect is used when the time period has not finished:



圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif
I have seen three movies this week.
(This week has not finished yet.)






圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif

The present perfect is often used when the time is not mentioned:



圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif
Gerry has failed his exam again.






圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif

The present perfect is often used when the time is recent:



圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif
Ikuko has just arrived in Victoria.






圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif

The present perfect is often used with for and since.



圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif
Greg has lived here for 20 years.
Greg has lived here since 1978.






The present perfect is formed like this:


圖片參考：http://web2.uvcs.uvic.ca/elc/studyzone/images/arro15e2.gif

have/has plus past participle
He ,she, it ( third person singular) has +past participle
We , I ,you have +past participle

Bye for now.

No. 2 is correct. I saw the boy singing. OR The boy was singing. OR
The boy sang.


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