Is it a strictly grammatical clause, or an idiomatic but ungrammatical short for 'If I die when I am young'?
If it is grammatical, is 'young' used as adverb, or 'die' used as copula, or a valid case of 'verb+adjective'?
Is similar structure common in real life conversations? Can you give me some more examples?
Yes, it is grammatical, and yes it means "if I die when I am young". "If I die young" is not a sentence by itself, but would normally be the dependent clause in a conditional statement.
Die is used as a copula in the sense that it is the primary verb linking the subject and predicate, but that's a pretty simplistic sense.
'Copula' usually refers to an existential 'linking' verb like 'be' rather than an active verb like 'die'. However, if you look at this answer, you'll see that although 'die' is not normally a copula, 'die' in "I die young" fulfills many of the criteria, particularly B and E. I'm not familiar enough with copulas to say for sure whether it is or isn't, but it looks close enough to me.
'Young' could be considered an adverb describing the way you would die, or as an adjective describing yourself at the time of death. I can see arguments both ways. 'Young' is normally an adjective, but [noun][verb][adverb] is the more common pattern. If I had to choose, I would call it a predicate adjective modifying 'I', as in the sentence "I am young" (where 'am' is a copula).
- "If I seem nice..."
- "If I go loud..."
- "If I run fast..."
- "If I learn quick..."
- "If I am fat..."
- "If I die handsome..." (the person was handsome at death)
- "If I die handsomely..." (the way in which the person died was handsome)
- "If I [almost any verb phrase you want]"
"young" in "to die young" is a normal adjective.The way someone can die can be peacefully or painfully, but the way of dying can't be young. "young" stands for "to die in young age" or "when someone is young". One does not think of the manner of dying but of the age of someone who died.
The school rule that after a verb follows an adverb is an oversimplification. There are a lot of verbs followed by an adjective where we clearly don't think of manner but of something else. But, of course, for beginners it is difficult to feel the difference and the grammar point verb + adjective is not a point in course books. It is advanced grammar.
to buy sth cheap (at a cheap price, not in a cheap manner)
to break free - He tried to break free (to get free)
to fly high - (at a high level or to a high point)
to ring true (that rings like sth that is true)