1. Simple word to be used in any situation, i.e. when calling a friend (preferably in a public place), as the subject of an email, when scribbling on a notepad.
Preferred use depends of the context
When written as a miscellaneous, unnecessary word in an email, use its 3l33t sp34k form, i.e d4L1.
When spoken, you should use a high-pitched town and say something like daaaaaaallliiiiiiiiii. Needs to be long as the most common use of it is to sound annoying (only towards one chosen person, who actually does not need to be present to use the word).
Better results are usually obtained when the word is used repeatedly by a group of people.
2. A question to which the answer should immediately be a fag.
3. A fag.
A: I'm sorry for not coming last night.
B: You're such a d4Li...
A: Haven't you had enough of saying that?
B: What, dali ? You're such a fag for not coming anyway
A: I know. Stop saying that !