First, you should ask if it is a formal or casual party.

= First, you should ask whether it is a formal or casual party.

首先, 你得問清楚它是個正式或非正式的派對。

上列句中, if引導的名詞子句作其前及物動詞ask的受詞。whether引導的名詞子句若作及物動詞的受詞時, whether可用if取代, 此時if譯成『是否』, 而非『如果』。

例: I don't know whether I'm right or not, but that's my
  　 opinion.

  　= I don't know if I'm right, but that's my opinion.

  　(我不知道自己對不對，但那是我的看法。)

  　＊ 用if取代whether時, 通常不與or not並用。

注意:

1. whether引導的名詞子句作主詞為虛詞it取代時, 此時whether亦可用if代替。

例: Whether the jury's decision was right or not is still
  　debatable.

  　= It is still debatable whether the jury's decision was right
  　or not.

  　= It is still debatable if the jury's decision was right.

  　(陪審團的判決是否正確仍有待商榷。)

但: If the jury's decision was right is still debatable. (╳)

2. whether引導的名詞子句若作介詞的受詞時, 此時whether不可被if取代。

例: Our departure depends on if the weather will clear up
  　 tomorrow. (╳)

  → Our departure depends on whether the weather will
  　clear up tomorrow. (○)

  　(我們何時動身要看明天天氣是否會放晴而定。)

3. whether引導的名詞子句若置於be 動詞後作主詞補語時, 此
  　 時whether亦不可用if代替。

例: The question is if the boss will approve this idea. (╳)

  → The question is whether the boss will approve this idea.
  　   (○)

  　(問題是老闆是否會同意這個點子。)