Spelling, Grammar, and Punctuation
No matter how good your spelling and grammar skills, you will need good reference books: a dictionary, thesaurus, and a stylebook or two (though no grant requires the use of one particular style over another). The Council on Foundations in Washington, D.C., has a stylebook particular to the philanthropy field, but it is geared more for those writing about philanthropy than to those writing to philanthropists.
Use, but don't rely on, spell check. Words can be spelled two ways, and while either will pass a spell check, only one is correct as it's used in your sentence. What could be more embarrassing than sending in a proposal from a “pubic” rather than a “public” school district?
If your readers are going to judge just the content of the grant, which is their most appropriate role, you must proofread for errors before submitting your first draft. Check tenses, check punctuation and run-on sentences, and check your spelling.
If you have enough time, walk away from your copy for a day or two. Then proofread it again — that's when mistakes will jump out at you. If you are rushed, ask someone you trust to proofread the document.

