The word ли is an unstressed interrogative particle. The use of this question word in yes-no questions is rather bookish; the way to ask yes-no questions in a more relaxed conversational style is to use the rise-fall intonation.
The intonation of questions with ли has a high falling pitch, as with other question-word questions (like Ктћ іто?). Compare the two ways of asking a yes-no question:
Сегћдня бџдет собрђние? Will there be a meeting today?
Бџдет ли сегћдня собрђние? (Same meaning.)
The most important word in the sentence is the one that has the rise-fall pitch in the conversational style and it is the one that comes first in the bookish style, with ли coming right after it. Compare the above pair of sentences with ones below, where the word сегћдня is the most important word:
Сегћдня бџдет собрђние? Is today the meeting?
Сегћдня ли бџдет собрђние? (Same meaning.)
The particle ли is used mostly in indirect questions rather than in direct questions, where it corresponds to English 'whether' (colloquially 'if'):
ВІ не скђжете, бџдет ли сегћдня собрђние? Could you tell me whether there's a meeting today?
ВІ не знђете, бџдет ли сегћдня собрђние? Do you happen to know if there's a meeting today?
Caution: Don't translate if as љсли when if means 'whether'. The Russian word љсли, unlike English if, is used exclusively in conditional clauses, like the first example below.
Ї емџ скажџ, љсли ћн придёт. I'll tell him if he comes.
Ї не знђю, придёт ли ћн.
(= Ї не знђю, придёт ћн или нљт.)
I don't know if/whether he'll come.