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I just would like to know if this sentence would be grammatically correct and in which cases should I use it:

I don't know when he is back instead of he will be back . I know the rule with when "no will" in most cases but we can say" please let me know when it will be available or when it is available"

If that's not correct let me know why please

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    "I don't know when he is back" is grammatical but you could better say it like "I don't know when he is coming back" – user6200 May 10 '15 at 10:25
  • "I don't know when he will be back" is grammatically correct because main clause in simple present tense is followed by sub-clause in the simple future tense. – Harish Chandra Rajpoot May 10 '15 at 11:21
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    Do not use I don't know when he is back. You can say I don't know when he gets/comes back, I don't know when he will get/comes back. – user6951 May 16 '15 at 14:40
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The rule you are referring to ("no will" with when) only applies in those sentences in which when introduces a time clause, e.g.:

*Give me a call when you will get my message.

Give me a call when you get my message.

In your example, when introduces a sort of reported speech, it is not a time clause. It is as if it were derived from direct speech along the lines of:

"When will he be back?" - "I don't know"

Therefore, it is correct to join the two sentences as:

I don't know when he will be back.

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