Help in learning English

I found a task in my book and decided to practice. I studied the rules and didn't understand anything. Could you do the task, I think that it will be easier to understand the finished one.

Exercise5. Complete the sentences with the correct forms of must, mustn't, have to, don't have to, can, can't, could, couldn't, should, shouldn't. Use the verbs in brackets.

1.He... chess last year but now he can (play). 2. You... and see your doctor about this terrible allergy (go). 3. Ann... her keys this morning (find). 4. Jim... at the airport at 9 a.m. to meet his friends (be). 5. People... litter on the beach (leave). 6. When you buy tickets for a plane, you... your passport (show). 7. ...me the way to the railway station (you/tell)? 8. He... mathematics very well when he studied at school (understand). 9. You... these dictionaries from the library (borrow). 10. Kate... this article for her report (translate). 11. I... where I had seen that person (remember). 12. We... a present for his birthday (buy). 13. He... the flowers - I'll water them later (water). 14. I.. to work today because my doctor said I should have a rest (go). 15. We... on time for the meeting or our boss will be discontented (be). 16. He... the guitar when he was little but now he can (play). 16. You... vegetables and fruit if you want to be healthy (eat). 17. She... an appointment with her dentist (make). 18. ...me to translate this article (you/help).