preposition "to" + indirect object

[Lenka]

When do you use an (or the??) indirect object with the preposition "to"?

I know that when you write the indirect object after the direct one, you always have to use "to": e.g. I'll send the letter to her.

If the indirect object comes before the direct one, you sometimes use the preposition "to" too, though. The only problem is that I don't know when.

Years ago you started writing to a penfriend.

You still write to each other regularly now.

Why was the preposition "to" used in the sentences? One does not always have to use "to" with the verb "write", does one? You can say "She wrote me (a letter)." and it is completely correct, isn't it? Why should one use "to" in the sentences above, then?
How do I recognize when I should use the preposition and when I should leave it out?

[2006]

When the indirect object comes first, you don't use "to". I'll send her the letter.
Regarding your two sentences in bold, which are a bit odd but could be possibe, "to" is 'optional' but I would use 'to" in the first one.

If you include a direct object, like 'letters', you need "to" in both sentences.

Years ago you started writing letters to a penfriend.
You still write letters to each other now.

[Buddhaheart]

Years ago you started writing to a penfriend. ‘Writing’ here means ‘writing letters’. Years ago you started writing letters to a penfriend. The preposition ‘to’ shows the relationship (action toward) between the noun ‘penfriend’ and the ‘letters’.

You still write to each other regularly now. Same explanation as above.

No, "She wrote me (a letter)." is old-fashioned and should be avoided. Correct usage should be "She wrote to me (a letter)." You should be aware this assertion is not shared by everybody.

The preposition is not restricted to "to’, there’re ‘of’, ‘for’, ‘from’ & etc. She bought a gift for me, for example .