Как правильно: he didn't understand, but the problem to him. - could -was able to -had can

Could Это правильное слово так как фраза переводится"не понимаю, но я МОГ  объяснить проблему ему.

Can будет. То есть "Он не понял, но я могу объяснить ему.

Dear Ashton,
I'm sorry I haven't written to you for so long, but I have been very busy lately. All last month I was having (had) exams and had to study very hard.
Anyway I have stopped studying now, and I am waiting for my exam results. As you see from my letter, my address has changed and I live in Leeds now.
My family moved here three months ago. I hope when you come to Britain in the summer, you will visit me. I will be at home till the middle of August. Then I am going on holiday to Scotland.
Hope to hear from you shortly.
Love,
Sarah

Dear Ashton,
I'm sorry I have not written to you for so long, but I have been very busy lately. All last month I had exams and had to study very hard.
Anyway I have stopped studying now, and I want (здесь что-то пропущено) for my exams results. As you see from my letter, my adress has changed and I live in Leeds now. 
My family family moved here three months ago. I hope when you come to Britain in the summer, you will visit me. I will be at home till the middle of August. Then I am going on holiday to Scotland. Hope to hear from you shortly.
Love, Sarah.