When would I advise asking a question on ELL, instead of ELU?
When the answer you seek is something that most native speakers could provide intuitively.
When would I advise asking a question on ELU, instead of ELL?
When you want a technical answer that delves into the detailed grammar involved the question.
As an example, I'll borrow this question from ELU, which (as of right now) has two close votes, and a few answers, too:
Could anyone explain what is the exact meaning for last long? When to use this?
For example: Every time I opened my Inbox there was a strange excitement. But most of the time it didn't last long.
The user Jony Agarwal has left what I would call the "ELL answer" to this question:
"Didn't last long" means it's something which is there for a short period of time.
Some synonyms are ephemeral, short-lived, brief, temporary, fleeting, transitory, transient, volatile, and momentary. There is also evanescent (mainly used in scientific excerpts).
Meanwhile, John Lawler has left what I would call the "ELU answer" to this question:
Long in the constructions take long, last long, be long -- all referring to extensive duration of an activity or event -- is a Negative Polarity Item (NPI). That means:
• Take long, last long, and be long are idiomatic; i.e, they don't follow ordinary rules. Synonymous phrases that do follow the normal rules are take/last/be a long time,
which are not NPIs.
• They are all ungrammatical, except when appearing in the scope of a negative term.
There are a lot of negative words, like doubt, forbid, prevent, seldom, only, and few, and even more negative constructions, like comparatives, questions, hypotheticals, etc. These are listed under "Negative Triggers" on the link above.
So, the key question is: What kind of answer was the O.P. seeking? A short explanation of the meaning of didn't last long, with a list of possible synonyms? Or a treatise on Negative Polarity Items?
It's clear which answer was appreciated more by the ELU community: Agarwal's answer is sitting at 0 upvotes after a few days, while John Lawler's answer has garnered a modest handful of upvotes.
That said, my gut tells me that the user really wanted the ELL answer to the question. John Lawler successfully transformed this ELL question into one more suitable for ELU by providing an ELU answer. But, really, I think the O.P. probably should have asked that question here on ELL.
Had the O.P. asked something more like this:
How come I can say She isn't going to be long, but She's going to be long doesn't sound quite right? Is there a grammatical term for this?
That's really more of the question John Lawler seems to be answering, and it's one that would probably be more suitable on the site primarily devoted to linguists and etymologists. But:
Could anyone explain what doesn't last long means?
seems like more of an ELL question.
Wherever you elect to ask your English question, remember: both sites expect you to do research first, and you should find a way to include the results of that research in your question.
For example, had this question been asked on ELL, I probably would have exhorted the user to at least say something like:
I looked up the words last and long in the dictionary, but I had trouble putting them together to understand the meaning of this phrase.
As it's written right now, I don't think it's a very good fit for either site, and neither the downvotes nor the close votes are unwarranted.