It's quite difficult to write a letter to a friend whose father is seriously ill and sounds not so good. I can think of nothing but "sorry to hear that" ,but that sounds not sincere enough. What else I could say, can I say "I'm writing to convey my sincere sympathy to..." Will that hurt too? I would appreciate your ideas! Thank you.
Just continuing ahava yin's post....Will it be correct if we write : "I was (am ?) greatly saddened to know about your father's ill health........" or "I was (am ?) extremely worried to know...." or " I was(?) deeply (or much) distressed to know....". I am not sure if these sentences are relevant or not. Could anybody please help?

/was (am ?)/ <<<--- means I don't know which is correct here- was or am? Please suggest the correct form.

1) I am greatly saddened to learn[or hear] about your father's ill health.......

Yes, that works.

2) I was (am ?) extremely worried to know....

Probably not. You are not worried. You might be concerned? [1] is definitely.

2a) I am concerned about your father's illness. [sounds a bit odd to me, but it is okay. We are usually not concerned about friend's family illnesses, but sometimes we are.]

3) I am deeply distressed to know....

Yes, that works, though it does seem dramatic.

I hope this helps.

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Thanks MountainHiker for your help!
It's very nice of you explaining the statements given above.
i was really shocked to know about your father's / mother's illness, May God give him/ her healt and a quick recovery and bring him/ her out of that stage soon. ....................