501. You'd better look before you leap．
502. You know what I'm talking about.
503. He has been sick for three weeks．
504. He inspected the car for defects．
505. I count you as one of my friends.
506. I go to school by bike every day.
507. I have a large collection of CDs.
508. I won't be able to see him today.
509. I'll call a taxi in case of need．
510. Is there any sugar in the bottle?
511. It's a secret between you and me.
512. It's very kind of you to help me．
513. Let's divide the cake into three．
514. Patience is a mark of confidence.
515. Susan is going to finish college．
516. That is my idea about friendship．
517. The book you ask for is sold out．
518. The boy was too nervous to speak．
519. The play may begin at any moment．
520. The salve will heal slight burns．