Business letters must always be formal, but we should write in a natural way to friends.
Business letters must always be formal, but we should write in a natural way to friends.
商业信函必须是正式的，但是写信给朋友应写得自然一些。
Business letters must always be formal, but we should write in a natural way to friends.
商业信函必须是正式的，但是写信给朋友应写得自然一些。