plain pin

例句：

1. 1. She used a plain pin to fasten the button on her shirt.
   她用一根直针把衬衫上的纽扣固定住。
2. 2. The plain pin was so thin that it easily broke off in her hand.
   这根直针太细了，在她的手里很容易就断了。
3. 3.He carefully placed the plain pin into the lock and turned it clockwise.
   他小心翼翼地将直针插入锁中，然后顺时针转动。