Write from your heart
If you want to build a relationship with your donors (a must if you want to keep your donors), you need to build the components of a friendship into your communications with your donors.
One of these components is colloquialism. What is colloquialism? It is just a big fancy word for not using big fancy words.
Forget about perfect college-paper composition when you’re writing a letter to your donors. Write from your heart. Connect to your donor using language she can understand.