Imagine reading a few sentences in a short paragraph, absorbing the meaning and grasping all the key points before reaching the end, and yet failing to notice a word was used twice within the text. On the second read, however, the repeated word is glaringly obvious. This occurs because your brain isn’t looking at every individual character. It looks for the next word and the intended meaning, fills in the small gaps, and moves forward. This process allows most readers to read quickly and efficiently, but it means the brain skips exactly the kind of details that a proofreader is trying to spot.
Try reading a passage of 100 words or so once, at your normal reading speed. Next, read the same passage over again, covering up the lines below the line you’re focusing on with a clean piece of paper, or using the on-screen focus guide to only highlight the one line at the time. Go back and mark all the typos, missing articles, double spaces, repeated words, and missed punctuation marks you see this time. You might be surprised to find that these were all there to see, but your eyes skipped over them when you were looking at the overall sentence meaning.
It’s easier to spot these mistakes when you weren’t the author of the piece, or if you didn’t read the passage before. When you wrote or read the text before, your brain tends to remember what it should have said, rather than what you actually saw in front of you. The missing article “the” doesn’t appear to be missing, since you know it was supposed to be there. Similarly, a word like “form” might not be picked up when “from” is needed, especially when both “form” and “from” are correct, and Spellcheck does not catch the spelling error. It doesn’t take much for the sentence structure to appear predictable, causing your eye to skim past errors.
Reading aloud often helps catch errors you otherwise wouldn’t because it gives your brain more sensory input, making it easier to hear what isn’t said, or read aloud the awkward punctuation. You might even read through your passage backwards, sentence by sentence, so that you aren’t looking for a logical or cohesive flow, but are instead checking for spelling and correct word structure. Changing the viewing method to enlarge text or change the page width, or even by printing a hard copy of the text, can make it feel less familiar. These don’t substitute for your proofreading ability, they simply help your eyes take a different path to your text.
Reading your text in separate passes can also be very helpful because you’re only looking for one specific thing at a time. Look only at spelling during the spelling check. Look only at the commas, quotes, apostrophes, and periods during the punctuation pass. You might want to check for capitalization consistency, hyphenation, abbreviations, the use of headings, and list punctuation during another pass. Trying to do all of the above in one go means your mind goes back to the same way it reads normal sentences, with meaning receiving the most attention rather than specific details.
A useful sign of progress here is that your text isn’t necessarily error-free on every page. You know you are getting better at proofreading when you recognize when you have begun to read the page, rather than checking it. When you realize you are just reading the page for meaning, stop, change your view of the text, and move on to checking for a specific, single purpose during the next pass. It’s easier to catch typos like these if you don’t let your sentence sweep you along.