I'll tell you when he arrives = I have something to tell you, I also want him to hear the news, so I'll tell you when he is here.
I'll tell you when he will arrive = He will arrive here at some point, and I'll let you know of his being here at that exact moment. |
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The first makes sense. But the second paraphrase would also expressed with "I'll tell you when he arrives", so you haven't completely understood, I think.
In both cases the "telling" is fixed in time by the meaning of the "when" clause. His arrival triggers the telling. So either is a possible paraphrase of "I'll tell you when he arrives."
Now, as for "I'll tell you when he will arrive": This would not normally be used for either of the meanings you paraphrased above. In this case I take "when he will arrive" as a piece of information, the mention of some hour of the day, for example. It says "I will inform you of the hour of his (yet to occur) arrival." And it doesn't actually say
when the informing will take place!!! Nevertheless you could add that: "I will tell you later when he will arrive".
Combining the two, I can say (albeit on a rare occasion!),
"I'll tell you when he will arrive when you sit down and listen"
which says, "At the time you sit down and listen I will tell you a piece of information, namely, the hour of his arrival". The sitting-down-and-listening-time triggers the telling, not his arrival time.
To complicate matters, the 'piece of information' idea is also possible in "I'll tell you when he arrives", which can also mean "I will inform you of the hour of his arrival".
So "I'll tell you when he arrives" is ambiguous.
"I'll tell you when he will arrive" is, in my opinion, not so ambiguous, since I only sense the 'piece of information' meaning here, not the 'trigger time' meaning. In English we just don't use the future tense in a when clause when we want the 'trigger time' meaning.
CJ