Letters written in a formal style often use the passive voice and contractions are not to be used. Informal letters, on the other hand, use active voice and contractions are generally used

I am writing to request a refund for a faulty television, purchased from your store last week. (Tone: Formal)

The writer can also convey tone through the words that are chosen to express ideas and attitudes. Each noun, adjective, verb and adverb can show the tone, which can be changed very quickly if the candidate has a wrong word choice

I am genuinely grateful for your invitation to the conference and I eagerly look forward to meeting you. (Tone: Formal)

I can’t wait to catch up with you and am looking forward to seeing you very soon. (Tone: Informal)