Object question

[nicolealp]

What is the grammatical explanation for when to use "me" or "to me"?
For example: Explain it again to me.
Example: He wrote me a terrible letter.

Ex: She sent a letter to them.
Ex: She sent them a letter

Do we use to + the pronoun when there is an object before the pronoun?

[2006]

What is the grammatical explanation for when to use "me" or "to me"?
For example: 1...Explain it (direct object) again to me (indirect object).
Example: 2...He wrote me ( in o) a terrible letter (d o).

Ex: 3...She sent a letter (d o) to them (i o).
Ex: 4...She sent them (i o) a letter (d o).

Do we use to + the pronoun when there is an object before the pronoun?

Yes, but the pronouns in your sentences are objects too; except for "it", they are indirect objects.

The correct way is as in your sentences.
When the indirect object (me, them) comes first, there is no "to". 2 and 4
When the indirect object comes after the direct object, there is a "to" before the indirect object (to me, to them). 1 and 3