Put in CAN or BE ABLE TO in the necessary form. Sometimes both variants are possible:
1) I have never … understand him.
2) Nobody … jump five meters high without a special gadget.
3) Will you … meet me at the station?
4) Will you show me how to do this jump? — I am afraid I … to show it today. My ankle won’t bend, I am afraid I will have to consult a doctor.
5) Has she lost weight? — Yes, she is a very strong-willed person. She … restrain from food even if it is very tasty.
6) Have you told her that her boy-friend dates Julia? — Not yet. I simply … to talk on such delicate topics.
7) … simply to hammer a nail into this wall? I am very disappointed.
8) He sometimes … do such weird things that people think he might be mental.
9) I have tried hard but, I am afraid, I … to do this. I will have to give it up.
10) He … speak four foreign languages. He … to do it since he graduated from the University.
Keys: 1) been able; 2) can; 3) be able to; 4) am unable; 5) can/is able (if we want to express our admiration of her ability); 6) can’t/am unable (if I want to stress my helplessness). 7) Aren’t you able to; 8) can; 9) am unable; 10) can; has been able;
For explanations see page "ENGLISH WORKSHEETS” | 
