是否
Whether (or not) (formal)
HSK 5HSK 3.0 · 5
(S) + 是否 + V/Adj | 不知道/决定/取决于 + S + 是否 + V
是否 is the formal counterpart of 是不是/V不V, meaning 'whether': 请问您是否需要帮助？. It is very common in embedded clauses (我不知道他是否同意) and in written notices. Since it already carries the question, do NOT add 吗. 是否 goes before the verb or adjective and is never combined with an affirmative-negative form (*是否去不去).
HSK 5 formal reading and sentence ordering.
Examples
我不知道他是否同意这个计划。
I don't know whether he agrees with this plan.
请问您是否需要帮助？
Do you need any assistance?
是否参加，由你自己决定。
Whether to take part is up to you.
这个方法是否有效，还需要研究。
Whether this method works still needs study.
Exercises
1.我还不清楚他 会来参加会议。
2.Choose the correct sentence:
3.“是否”最接近的意思是：
4.这种药 有副作用，还要做进一步的研究。