是否shìfǒu

Formal Adverb 是否 (shìfǒu): Whether or not

Meaning: Whether or not (formal equivalent of 是不是)

Pattern

是否 is the formal written equivalent of 是不是 ("whether or not").
It most frequently appears in embedded clauses following verbs of inquiry, cognition, or decision such as 知道, 询问, 讨论, 决定, 看, or 取决于 (e.g., 我不知道这个消息是否真实 — "I don't know whether this news is true").
Because 否 already means "not", you should NEVER add another negation word like 不 after 是否 (*你是否不想去 is redundant; say 你是否想去).
In casual daily conversation, 是不是 or V不V is preferred over 是否.

1
请告诉我，你是否决定参加这次比赛。
Qǐng gàosu wǒ, nǐ shìfǒu juédìng cānjiā zhè cì bǐsài.
Please tell me whether you have decided to participate in this competition.
2
他是否会成功，还要看他的努力。
Tā shìfǒu huì chénggōng, hái yào kàn tā de nǔlì.
Whether he will succeed still depends on his own efforts.
3
我不知道这个消息是否真实。
Wǒ bù zhīdào zhège xiāoxi shìfǒu zhēnshí.
I do not know whether this piece of news is true.

•Since 否 already means "not", do not add 不 after 是否; used in formal or written contexts.

⚠️ Common Mistakes:

•Redundantly adding 不 after 是否 (*你是否不想去 instead of 你是否想去).

•Overusing formal 是否 in casual everyday small talk where 是不是 or 吗 is natural.

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