Dew and do are easily confused words. They are also homophones. This means they sound the same, but have different spellings, and different meanings.
The spell-check application of most word processing software programs would not catch a slip-up of these two words. Spell-check is looking for words that aren’t in its dictionary, and words that resemble words in its dictionary but are possibly spelled wrong. Spell-check isn’t perfect. It doesn’t know and can’t guess what word you wanted or what word you meant, it can only judge the words on the page. If you used words that are all spelled correctly, it gives you a pass anyway.
Autocorrect suggests words that start with the same letters. It’s suggesting what word you may want to save time, but quite often, its suggestions are pretty off base. They don’t help you out, but they do make you laugh.
Dew (pronounced “doo”) is a noun. It means the condensation that forms on land and other surfaces as night becomes day.
Do (pronounced “doo”) is a verb, it means to perform a task or activity.
The following story uses both words correctly:
Domingo had just gotten out of school the day before. He rode bikes, flew kites, everything that he dreamed of doing while in school. By the next morning he was bored out of his mind. He had run out of all those things to do, he was watching the dew disappear from the furniture on the back porch.