【English】Useful phrases for writing letters of advice

To offer sympathy (usually used in opening paragraphs)

I'm sorry to hear about your problem.
I'm sorry to hear you're not enjoying school.
I think you have a very difficult problem, but I hope I can help you.

Other useful phrases used in opening paragraphs
My opinions might be a bit one-sided. You must of course, use your own judgment so that whatever you decide to do will be best for you.

To give opinions

I think you should talk to your sister.
I don't think you should ignore your friend's advice.
In my opinion, you ought to tell your friend.
I'm sure your friend will understand if you say no.
As far as I can tell, there's no need to worry about upsetting him.
The best idea is to concentrate on your studies.

To make suggestions 

You could try going to dance classes.
You should try to learn a new language.
You shouldn't visit him unless your parents agree.
You must ask for your parents' permission.
You mustn't use your real name on ICQ unless you know who you're chatting with.
You ought to go to your grandfather's dinner.
I suggest you talk to an adult you trust.
I'd advise you to go to your cousin's party.
My advice would be to go to the concert.
My advice is to explain the situation to your friend.
If I were you, I'd go jogging every evening.
Why don't you try talking to your older sister.

To express hope 

I hope that this advice is helpful.
I hope you solve the problem soon.
Hopefully, things will get better soon.
I hope this advice is useful to you but remember that no one but you can make this decision. Whatever you decide will, I am sure, be the best for you. 

To express interest in the outcome

Please write and tell me how you get on.
Please get back to me and tell me what happens. 
Let me know how things turn out.