где ошибки?

Моя версия

1. If you want to pass your exam, you should work harder.
2. You shouldn't wear a suit and a tie.
3. It can't be Rachel's birthday today. The party suppose to be next Saturday.
4. Yesterday Karl went to dantist.
5. If you want to improve football skills, you ought to practise more.

1. If you want to pass your exam, you can work harder.
   if you want to pass your exam you must work harder
You do not (don't) need to wear a suit and tie.
Rachel's birthday can't be today. The party is next Saturday. (как ариант. не уверен)
Yesterday, Karl had to go to the dntist
If you want to improve your football, you ought to practise more often.

отсебятина. мб кто проверит, буду рад узнать ошибки =)

If you want to pass your exam, you can will work harder. или просто work harder (смотря что хочешь сказать:ты будешь работать или тогда работай)
You needn't to wear a suit and tie. (не уверена)
It can't to be Rachel's birthday today. The party is next Saturday.
Yesterday, Karl must have gone go to the dntist. (но не уверена опять же)
If you want to improve your football, you ought to practise more often.