"if"と"whether"の使い分け

・"if"と"whether"は名詞節を導き、｢～かどうか｣という意味で使われ、置き換えて使える場合が多い。

【正】It is not clear whether you can help solve my problem.
【正】It is not clear if you can help solve my problem.
【正】We didn't know whether he was dead or alive.
【正】We didn't know if he was dead or alive.
【正】I doubt whether anyone will remember me.
【正】I doubt if anyone will remember me.
【正】I wonder whether you could help me.
【正】I wonder if you could help me.

・”whether”は主語となる名詞節を導くが、"if"は主語となる名詞節を導けない。
【正】Whether you like it or not doesn't matter.
【誤】If you like it or not doesn't matter.
【正】It doesn't matter whether you like it or not
【正】It doesn't matter if you like it or not.

【正】Whether she comes tomorrow or not is what I want to know.
【誤】If she comes tomorrow or not is what I want to know.
【正】What I want to know is whether she comes tomorrow or not.
【正】What I want to know is if she comes tomorrow or not.

・"if"が導く名詞節は前置詞の目的語になれない。
【正】Americans were divided on whether the country was better off under Obama.
【誤】Americans devided on if the country was better off under Obama.
【正】Scientists look at whether climate change is causing bigger ocean waves.
【誤】Scientists look at if climate change is causing bigger ocean waves.
【正】This is the key to whether your customer is satisfied with you.
【誤】This is the key to if your customer is satisfied with you.
【正】There are doubts about whether he will come.
【誤】There are doubts about if he will come.
【正】She raised doubts about whether his mother told truth.
【誤】She raised doubts about if his mother told truth.

・"if"が導く名詞節は"discuss"の目的語になれない。
【正】We discussed whether U.S. troops should stay in Okinawa.
【誤】We discussed if U.S. troops stay in Okinawa.

・"if"が導く名詞節は"consider"の目的語になれない。
【正】We need to consider whether we are really solving the problem.
【誤】We need to consider whether we are really solving the problem.

・"if"は不定詞句を導けない。
【正】She can't decide whether to marry him.
【誤】She can't decide if to marry him.
【正】She can't decide whether to tell him the truth.
【誤】She can't decide if to tell him the truth.

"if"と"whether"の使い分けの関連項目

• ⇒ and, but, or, so
• ⇒ that - 接続詞
• ⇒ whenとwhile
• ⇒ because
• ⇒ as～as…
• ⇒ if
• ⇒ after／before
• ⇒ since／until
• ⇒ than
• ⇒ Immediately you begin to speak, he gives you his full attention.
• ⇒ Robert left whilst the show was still ongoing.