half an hour,是一個整體概念,自是單數
例句:
It'll be ready in half an hour's time. 
另,4one and a half +名詞"作主語時,謂語動詞要用單數.例如:One and a half apples is left on the table.桌子上有一個半蘋果.
half an hour在英語裏的意思是半個小時
半個小時是30 miuntes ,所以是單數
複數名詞表示時間,價格等時,表示單一的概念,謂語動詞要用單數形式.
例句:
Three months is a long time.
   Ten minutes seems an hour whe one is waiting for a phone call