"I wrote a letter to Peter."
Translation:मैंने पीटर को एक चिट्ठी लिखी।
Bit late on a reply but would like it on this thread regardless, मैं or मैंने depends on whether the verb is transitive or intransitive, that is to say whether it takes an object or doesn't. "To write" is transitive as it takes an object, you can write something, a book, a letter, etc, unlike intransitive verbs which take no object, like "To go," you cannot "Go something." मैं is used with intransitive verbs (I went = मैं गया,) and मैंने is used with transitive verbs, as in this sentence. Though keep in mind that मैंने is only ever used in the past simple tense.
As far as I know, as a non-native but who has studied a lot of Hindi, that would not be considered proper Hindi.
However, in Punjabi (which I know better than Hindi), it WOULD be just "main". And since "Punjabi" language blends into a spectrum of languages until we reach "Hindi" of Delhi... I would not be surprised if spoken dialects of Hindi do the same.
Will be interested to hear what others say.