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Hello,

1. I've read that the verb after 'knew' should be in the past tense. Is this always true?

I knew that man landed on the moon.

2. Can I also say 'I knew that man has landed on the moon'?

3. I've also read from a thread here that 'did you know...?' can take a verb in present tense after it because it is equivalent to saying 'Are you aware that...?'. Therefore, is it correct to answer the below question in present tense as follows?

Did you know that the earth is round?
Yes, I knew that the earth is round. (Or it should have been 'was'?)

4. To my ear, changing to 'was' doesn't sound right because the statement is a general truth. 'Was' sounds it is no longer true. Would you agree?

5. Also, don't you think it's more correct to answer 'Yes, I know that the earth is round'? What I'm not sure about is it doesn't match the tense of the question in past tense, although it is equivalent to a question in present tense 'Are you aware that...?'.

6. In the first place, what is the difference between the two sentences below?

Did you know that the earth is round?
Do you know that the earth is round?

I would very much appreciate your assistance in this.
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Anonymous1. I've read that the verb after 'knew' should be in the past tense. Is this always true?
No. You have given some execllent examples of when that is not true.
Anonymous'I knew that man has landed on the moon'?
It sounds OK to me. But past perfect is much more common:

When I returned from my long sailing vacation in the summer 1969, I knew then that man had landed on the moon.
When the battle was over, they knew that they had lost the war.
AnonymousDid you know that the earth is round?
Yes, I knew that the earth is round. (Or it should have been 'was'?)
The last time I checked, the earth had not changed its shape. It was round, it is round and, unless it is hit by a very big asteroid, it will be round. For these universal invarient facts, present tense emphasizes the invarient nature of that fact:
From observing lunar eclipses, the ancients discovered that the earth is round.
Even when I was a young child, I knew that water boils at 100 degrees celcius and freezes at 0 degrees celcius.

You will often hear the past tense - that is also OK, because the fact was equally true in the past.
Anonymous1) Did you know that the earth is round?
2) Do you know that the earth is round?
The difference is that #1 is asking about what I knew in the past (and if I haven't forgotten, I still know it now!), and #2 is asking about what I know right now.
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Thank you so much for your clear explanations. That makes sense now.

Just a few follow-up questions...

1. In informal conversations, is it correct to reply to the question 'Did you know that the earth is round?" with the following answer not having the same tense?

Yes, I know that the earth is round.

2. Or the reply should be 'Yes, I knew that the earth is round'?

3. But in formal writing, tense-matching should be strictly observed?
Anonymous1. In informal conversations, is it correct to reply to the question 'Did you know that the earth is round?" with the following answer not having the same tense?

Yes, I know that the earth is round.
Informally, people often use the past "did you know..." instead of "do you know". It shades the meaning a bit - asking about prior awareness.

The answer is commonly present tense. It shades the meaning to current awareness and caring about it at present.

Did you know that Joe and Angela are getting a divorce?
Yes. I know. (and I care about it)

Did you know that Joe and Angela are getting a divorce?
Yes. I knew that. (but I forgot about it because it is not important to me)
Anonymous2. Or the reply should be 'Yes, I knew that the earth is round'?
You are implying - Yes, I studied that in school, but until you reminded me, I had forgotten about it.
Anonymous3. But in formal writing, tense-matching should be strictly observed?
Not in the case of invarient facts. (e.g. my example about the ancients who discovered that the earth is a sphere.)
Thank you very much indeed. You've explained it very well. I can now see the subtle difference between their usages.
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