31 Derek was caught stealing money at school; the principal told him that he was not allowed to go back to school _____his behavior improved.
(A)since
(B)only if
(C)until
(D)provided that

provided, provided that, providing, providing that等是表示「條件」的連接詞，用以引導附屬子句，意思是「如果~，在~的條件下」，屬於文語用法。例如：

They may swim provided (that) an adult accompanies them.
(如果有位大人陪伴，他們就可以游泳。)

Providing (that) there is no opposition, we shall hold the meeting here. (如果無人反對，我們將在這裡開會。)

Derek was caught stealing money at school; the principal told him that he was not allowed to go back to school _____his behavior improved.

Derek在學校被抓到偷了錢，校長告訴他不允許回到學校，_____Derek改善行為前。 

(A)since

(B)only if

(C)until

(D)provided that

Until「直到...時，到...為止」，不過要注意 until 是用在表示「一直做某件事直到某個時間」。

1. I lived in Taitung until I was 12 years old.
2. This ticket is valid until December.
3. I will not be here for five days, but I will not be back until Saturday.
   

Only if｜只有…才

1. Only if是一個限制條件句，表達唯一的條件，像是「只有…才能…」、「只有在…的時候，才能…」或「除非…才會…」
   
2.  I will do it for you only if you ask me nicely.
3. 除非你好好的問我，我才會幫你做這件事。

If only｜但願

1. If only其實就是If的加強語氣，可以同時表達講者的願望與條件
2. If only I didn't have so much homework I could go to the party tonight.
3. 唉，要是沒那麼多作業我就可以去今晚的派對了。
4.  If only I had a girlfriend.
5. 但願我有個女朋友就好了。

Derek was caught stealing money at school; the principal told him that he was not allowed to go back to school _____his behavior improved.

(A)since

(B)only if

(C)until

(D)provided that

Derek在學校裡偷錢被抓。 校長告訴他，不允許他上學_____他的行為有所改善。

（A）自從

（B）僅在

（C）直到

（D）提供