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"I knew he must have been about seventeen"- Is it correct sentence?

I have a confusion with the sentence given above. Because as far as I know that if the main clause is in past tense then the subordinate clause must be in past form. If it is correct, I need to know why. Please some body help me to solve this problem!

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    There is no such rule. Your sentence is fine. It's perfectly correct to write things like: She said (past tense) that he is on his way (present tense) and that he will arrive tomorrow.
    – Ronald Sole
    Commented May 22, 2018 at 19:44
  • @RonaldSole Is it ok to write "I knew he is on his way"?
    – Araucaria - Not here any more.
    Commented May 22, 2018 at 21:43
  • @Araucaria If you were asked when you learned that someone was due to arrive the following day, you might reply: I knew (yesterday that) he is on his way. Or if a teacher asked pupils which of them knew that he is due to arrive shortly, one of them might reply: I knew (that) he is on his way.
    – Ronald Sole
    Commented May 22, 2018 at 22:18
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    "Must " in deduction doesn't change, "must" in obligation might change into "had to".
    – V.V.
    Commented May 23, 2018 at 5:03
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    No, I just said when you use "must" to express deduction, it doesn't change after the main clause in past tenses.
    – V.V.
    Commented May 23, 2018 at 8:05

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"At that time, I knew (that) he was seventeen." -> "I knew he was seventeen."

These should look fine to you, as they're grammatically correct.

Let's look at the colloquialism 'must have been'. By using 'must have been', the author of the lyric is trying to insert uncertainty.

"he was seventeen" = there's a male, age 17. "he must have been seventeen" = there's a male, probably age 17.

'about' just further enhances that uncertainty by making it "approximately 17": "he must have been about seventeen" = there's a male, probably age 17 (approximate).

So putting this together: "I knew he was 17" = I'm certain he's 17. "I knew he must have been about 17" = I'm pretty sure that his approximate age was 17.

(Also, take note that it's a song lyric, so some colloquialism may be there for meaning and some for scansion.)

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  • Thank you so much! Are you a native speaker?
    – Abdullah Omar Nasif
    Commented Jun 2, 2018 at 7:59
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    @Omar Yes, I'm a native speaker.
    – JKreft
    Commented Jun 4, 2018 at 1:45

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