Sometimes as we're busy getting the words down in a manuscript, little errors can creep in that we don't notice. Most of the time, Word or whatever processor/program we're using will pick up on it, but sometimes, like at the end of sentences, it will sneak past the built-in grammer checker, too.
The problem: spaces
E.g. This is the first sentence. Between this and the first sentence, there are supposed to be two spaces. However, there are three. Visually, you can hardly tell there's an extra space there, but it's there.
E.g. This is the second sentence. Between "the" and "second" there are two spaces, when there should only be one.
These are minor, but attention to detail can put you ahead of the game with editors, who have to look at enough bad grammar and punctuation without having to deal with a manuscript that doesn't look quite right.
The fix: "Find"
The "find" feature (in Word you press alt-E, F) will allow you to find those extra spaces. Enter two spaces in the "find" box and then click on "find next." It will put a small black rectangle between every sentence. If the rectangle moves on to the next sentence, you have exactly two spaces after the period, question mark, or exclamation point. If, however, the rectangle just moves slightly and stays between those two sentences, you have an extra space. You can eliminate that with either backspace or delete.
The same holds true for extra spaces between words. Most times, the internal grammar checker will underline those in green (in Word), alerting you to the problem, but sometimes it doesn't. This will show you where those mistakes are while you're busy checking to make sure you don't have extra spaces at the end of your sentence.
Another good reason for looking for extra spaces: sometimes you can fit more onto a page and avoid "widows and orphans," as Word calls those pages with a single word or sentence at the top and the rest of the page blank. Think of it as one more way you can line edit more easily. Happy editing!