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Look at this sentence:

I knew that the sky is blue.

To me, it doesn't make sense to say the sky was blue since it still is blue. Even so, is it grammatically incorrect to say I knew that the sky is blue just because the tenses must be the same?

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Saying I knew that the sky is blue is correct, assuming the sky is always blue. It is fine to mix the present tense with the past tense when one is expressing a timeless fact. Of course, it is also correct to say I knew that the sky was blue, but that only expresses how the sky was blue back then.

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(Yesterday) "I know the sky is blue" said 3125.

(Today) 3125 said he knew the sky was blue.

Your exact words when you spoke yesterday were; "... the sky is blue". However, the time when you uttered those words was in the past i.e. yesterday. You cannot express your thoughts in the past using a present tense. The present tense is also used to speak about facts which are always true e.g; The sun is a star in the centre of the solar system. Water freezes at 0°C. Tuesday comes after Monday. Australia has kangaroos etc.

So you should say: "I know the sky is blue" if it is a fact and you know the sky is always blue. If instead you say,

I knew the sky is blue

you are in fact stating that yesterday you knew without a doubt the sky is blue. Since then however, your certainty has changed and effectively speaking, you don't know if it is true any more (yesterday you knew the sky is blue but today you don't know). The sentence is grammatically correct but it struggles to make logical sense.

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  • I don't think that the certainty has to have changed. If, for example, you recall something and want to make a point of the fact that at that time you already knew that the sky is blue (and might still know that), this works perfectly fine. "I was only four years old at that time. He was trying to make me believe the sky is originally red, but I knew the sky is blue." Or, you could use it as an idiom for somebody trying to make you believe something, but common sense tells you it is wrong. So the blue sky is just something, everybody knows.
    – skymningen
    Commented Aug 13, 2013 at 10:00
  • I disagree. When referring to the past and specifically retelling an anecdote, a story or an episode you need to use the past tense. In your example it would be far more natural to say: I was only four years old at that time. He was trying to make me believe the sky was originally red, but I knew the sky was blue. This structure is called reported speech or indirect speech.
    – Mari-Lou A
    Commented Aug 13, 2013 at 10:16
  • You are only reporting the words said by another person, if it is direct speech, then it would be legitimate to write: I was only four years old at that time. He was trying to make me believe the sky was originally red, but I replied indignantly, "I know the sky is blue!"
    – Mari-Lou A
    Commented Aug 13, 2013 at 10:18
  • You're right, I should have written " He was trying to make me believe the sky was originally red". But I still think "I knew the sky is blue" can be correct after that. I knew it then, it still is blue now.
    – skymningen
    Commented Aug 13, 2013 at 12:39
  • @skymninge I beg to differ. I knew the sky was blue means at a specific point in the past you held this belief, it does not necessarily follow you have since changed your idea. Let us pick another example, "When I was 10 I loved reading books." Does this exclude the possibility of my enjoying books later in life? No, why should it? If however, I say: "When I was 10 I used to love reading books" it would be correct to presume today I no longer read or enjoy books. Likewise: I knew the sky used to be blue OR I used to know the sky was blue means today it is no longer true.
    – Mari-Lou A
    Commented Aug 17, 2013 at 21:39

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