Letters of condolence
Letters of condolence are personal letters addressed to the bereaved. You send them to let the bereaved know that you have heard of the death, and to express your sympathy. They are a courtesy as well as a kindness.
These are problably the most difficult letters you will ever have to write. But do not allow that to put you off. Letters of condolence are always a great comfort to the bereaved. They show that there are people out there who care, and who are thinking of them.
What do I write?
Say that you are sorry to hear of the death, and that you send your deepest sympathies. Even just that may be enough.
But it helps to add a little more. Perhaps tell the bereaved a little about what the deceased meant to you. You could perhaps include some memories of them, or mention an anecdote that you are particularly fond of. This can help to spread the burden of grief.
Do not worry at all about your writing style, or feel that there is some kind of formal style that you have to adopt. Write as you would speak: with sympathy, with your own personal thoughts and words.
When?
Write as soon as your hear about the death. Do not delay. The task will not get any easier.
Just send an email?
No, not if the person who has died was in any way close to you. Letters of condolence should be written on paper and posted in the mail. They are treasured as physical objects, and are quite likely to be passed around among friends and relatives of the deceased, and kept for the record. Memories and anecdotes mentioned in your letter may even be used in the funeral eulogy.
Email may be OK, however, for more distant deaths, for instance if a friend or colleague loses his or her mother, whom you have never met. In such cases a posted letter may seem over-elaborate, and may put the recipient under the awkward obligation to answer.
Should I expect an answer?
No. The bereaved may be far too preoccupied to respond. In fact, you could mention in your letter that you do not expect an answer, for that reason. In due course, however, the bereaved may like to respond, to thank you for taking the trouble to write.