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When I grow up, I am going to be a doctor.

Is this sentence correct? Personally I think my growing up will happen in future and "I am going to be a doctor" sounds like it is going to happen in next month! But the sentence clearly indicates the speaker is a child. So according to me, "I am going to be a doctor" should be replaced by "I will be a doctor". So I think a more clear sentence would be

When I will grow up, I will be a doctor.

Is my concept correct here?

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    I think you may be confusing I am going to with I am about to. – StoneyB on hiatus May 13 '13 at 12:57
  • Your sentence "When I will grow up, I will be a doctor." is absolutely wrong. "When I will grow up" Should either be "When I grow up" or "I will grow up" – SovereignSun Mar 24 '17 at 10:46
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I don't think your concept is correct. The sentence is correct as written. There is no different "distance into the future" implied by the use of going to be versus will be. I can use either to talk about things that will happen in the next 2 minutes and the next billion years:

I am going to finish this answer before I go to bed.
The Milky Way and the Andromeda galaxies are going to collide in about 4 billion years.

Going to may imply more of a "on a path, trajectory or plan to achieve something" whereas will just indicates futurity.

  • "Will" can refer to a dream here. When I grow up, I will be a doctor. - may mean that the person wants to become a doctor it doesn't mean that he finally will. When I grow up, I am going to be a doctor. sounds like he is sure of him becoming one. – SovereignSun Mar 24 '17 at 10:45
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"When I will grow up, I will be a doctor." should be written as "When I grow up, I will be a doctor." to be idiomatic English. (See Using the future tense in a sentence containing a dependent clause starting with "when"​.)

"Be going to" is used to show what somebody intends to do in the future, but that doesn't necessarily means between five minutes.

We are going to buy a house when we have saved enough money.

It is also used to show that something is likely to happen very soon or in the future.

I think I am going to faint.

Will is used to speak about future or predicting the future (among other uses).

By next year all the money will have been spent.

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    Idiomatic means "natural to a native speaker". Children where I grew up will invariably say "When I grow up, I am going to be a doctor" instead of "When I grow up, I will be a doctor." That seems to be true even if you phrase the question "What do you want to be when you grow up?" (Verified by asking my daughter.) – Mike Sherrill 'Cat Recall' May 13 '13 at 13:43
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    I was not referring to the fact the OP is using "I am going to" instead of "I will"; I was referring to the fact the OP wrote "When I will grow up, I will be a doctor." – kiamlaluno May 13 '13 at 13:49

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