是否【shìfǒu】

是否 means "whether" or "whether or not", and it is the formal version of 是不是. It goes right before the verb or adjective it asks about.

In a direct question

是否 can ask a question on its own, in forms, notices and polite service language. 是否 already does the asking, so 吗 is left off.

• × 您是否需要帮助吗？ → 您是否需要帮助？ / 您需要帮助吗？

"Whether X" as subject or object

是否 + verb phrase can act as a noun phrase: "whether X". Verbs that take it include 知道, 确定, 决定, 考虑, 检查 and 取决于 "depend on".

Before a noun, keep 是

是不是 can stand before a noun – 他是不是老师 – because 是 is the verb. 是否 is not a verb, so a noun still needs 是 or the written 为 after it.

• × 他是否老师？ → 他是否是老师？

能否 "whether one can" works the same way with 能. …与否 "whether or not", at the end of a phrase, has its own entry.