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(1)I have already had my lunch at six o'clock.
(2)I have had my lunch at six o'clock.
(3)I have had my lunch, so I feel very full now.
(4)I have had my lunch at six o'clock,so I feel very full now.
(5)I had my lunch at six o'clock.
(6)I have had my lunch.
Could anybody check my sentences and my ideas on present perfect and past tense?
I think when our sentences contain the meanings of already we can use perfect for example.
Ali: Don't you want to go for a lunch?
I:(1)I have already had my lunch at six o'clock.
(2)I have had my lunch at six o'clock.
(3)I have had my lunch, so I feel very full now.
(4)I have had my lunch at six o'clock,so I feel very full now.
(5)I have had my lunch.
I can answer him with any sentence of the five sentences.
Many Thanks in Advance
Victory Ong
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As a general rule, do not mention a specific time that the action ocurred. "Already", as you mentioned, is fine; "at six o'clock" is too specific.

You would say, however, "I have had my breakfast at 6:30 every morning ever since I can remember", the governing time reference being "since I can remember".
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Could you simplify your explain?
I don't understand your sentences!
I think if you tell me whether which sentences are correct then I understand.
If I get others think which I don't understand ,I will ask you then.
Many Thanks in Advance
Vctory Ong(1)I have already had my lunch at six o'clock.

As Philip has already explained, as a rule, present perfect do not require "time stamps".

(2)I have had my lunch at six o'clock.

(3)I have had my lunch, so I feel very full now.

(4)I have had my lunch at six o'clock,so I feel very full now.

(5)I had my lunch at six o'clock.

(6)I have had my lunch.

First 6 o'clock is too late for lunch. For the above, I would simply say "I already had breakfast at 6 o'clock"

Could anybody check my sentences and my ideas on present perfect and past tense?

I think when our sentences contain the meanings of already we can use perfect for example.

Ali: Don't you want to go for a lunch? Any one of the following is fine for an answer.

I:(1)No, thanks, I have already eaten.

No thanks, I've just had lunch

Or, simply "NO thanks, I just ate"

(2)I have had my lunch at six o'clock.

(3)I have had my lunch, so I feel very full now.

(4)I have had my lunch at six o'clock,so I feel very full now.

(5)I have had my lunch.

I can answer him with any sentence of the five sentences. - You may still be understood by any one of the above. However, grammatically and logically some of them are not quite natural. I had commented on your past perfect useage for the same reason.

I think you have picked the right tense for the wrong contexts.

Many Thanks in Advance

Victory Ong

It's said that past tense is used for action that happened in the past and the action is irrelevant to now.
It's said that present perfect tense is used for action that happened in the past and the action is relevant to now.
Could anybody share with me that how they differentiate whether an action is relevant to now, or irrelevant to now?

Many Thanks in Advance
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Vctory,

Many examples have been used and posted here in response to your questions. If you Google the two tenses, you will find all the answers that will clear all your doubts. But I will give you a few from the top of my head.

I went to LA last weekend. - simple past.

I drove nearly 700 miles on that trip - past

I had a flat tire driving back- past.

I have heard a lot about Las Vegas but never been there. - present perfect. This type of context does not require "time reference". It simply tells people that you had heard about it since time passed and you still have periodically.

Same with " I have heard and seen enough of gossips about Michael Jackson ".