Future tense usage: “When you see it …”
When he will arrive, he will call you.
Please let me know if the tense and syntax in this sentence are correct. If not, then why not?
The fragment "when he will arrive" does not carry the proper tense in this case. Proper alternatives would be:
or, better yet:
The original sentence does not work because of mixed tense. "He will call you" is a statement of fact, and the word will indicates that the call is going to happen at some time in the future. "He will call you" has future tense. To specify the time at which the call is going to take place, you would say: "He will call you [at 4:00]" or "He will call you [tomorrow]". I used this same construction to come up with my sentence #2: "He will call you [when he arrives]". "When he arrives" indicates an exact time. Sentence #1 is a simple re-ordering of the two phrases in sentence #2. The two sentences have the same meaning.
When he arrives, he will call you is the correct form, as e.James says. It may seem like this is wrong because arrives is present tense while will call is future tense (instead of seemingly agreeing, as in your question). But English uses this pattern for future conditionals, as this structure is called. The conditional part (after the when) is given in the present tense, while the resulting phrase is in the future tense. This page has a good explanation of this form.