Most people learn languages by hearing other people speak and trying to imitate them. We learn written languages much later, when we go to school and learn how to read.
It’s an efficient system that works well most of the time – but it can also create problems when we have to write letters and endings that our ears don’t notice.
Here are a few examples:
Many people forget the s in lists, firsts, firsts, and similar words. (Say them aloud and you’ll hear what I’m talking about!)
The latest newsletter lists several job openings that interest me. CORRECT
And many people forget the –ed in supposed to and used to. (Again, listen to yourself say them aloud.)
Although we used to discourage women from applying, we’re supposed to actively recruit them now. CORRECT
Your ear can create other kinds of difficulties too: Misspelling words (many people forget the middle c in Arctic and the i in foliage); punctuation errors (often you can’t hear the difference between a comma and a period), and sophisticated usage that you don’t often hear in everyday conversation–agreement issues for pronouns and verbs, for example.
Bottom line: Educate yourself. Reading is a wonderful avenue to better writing, and you don’t have to limit yourself to grammar books. Any good magazine or book will expand your knowledge of language.