SenseiHanzo
at 2015-04-03 02:21:41 UTC
I don't think it strictly goes off the next syllable. For instance, in 何台 it's なん, but in 何だらだらしているんだ it's なに.
Here's one answer I found: http://japanese.stackexchange.com/a/3517
The Jisho entry already specifies that なん is the one used in counters.
Kimtaro Admin
at 2015-04-07 13:28:45 UTC
One thing I plan on adding to Jisho is better usage explanations, like なん vs なに. I've also started linking to great external explanations, and plan on linking to many more, for example good StackExchange discussions.
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Nani vs nan
I reckon it would be a good idea to explain the difference. In the dictionary it just mentions nan as "other form".
I myself know that the usage of each depends on what is the next syllable. But I don't remember which goes with which.