You’ve watched your child read “caterpillar” out loud without skipping a beat. You felt that little swell of pride. Then two words later: “of.” A pause. A substitution. They say “or” instead, or just barrel past it and hope you didn’t notice.
How does that happen? How can a kid decode a six-syllable word and then stumble over a two-letter one? If you’ve been wondering whether this means something is seriously wrong, or whether your child just isn’t trying hard enough, I want to stop you right there. This pattern is one of the most common things I hear from parents of kids with dyslexia, and the explanation has everything to do with how reading works in the brain, and nothing at all to do with intelligence or effort.
What Function Words Actually Are
Words like “the,” “of,” “by,” “so,” “and,” “to,” “is,” and “for” are called function words. You’ll also hear them called high-frequency words or sight words, though those terms aren’t quite the same thing. The reason function words are so tricky isn’t about how they sound or how they’re spelled. It’s about what they mean.
Or more accurately, what they don’t mean on their own.
Think about the word “caterpillar.” The second you see it or hear it, your brain pulls up a picture. A fuzzy bug. Maybe a leaf. “Caterpillar” has a concrete referent, something your brain can attach it to. Now think about “of.” What image comes to mind? Probably nothing. “Of” is abstract. Its meaning shifts depending on what surrounds it. “A cup of coffee” and “afraid of heights” and “the end of the road” all use the same two-letter word, and it functions differently in each one.
This is exactly why function words are so difficult to anchor in long-term memory for kids with dyslexia.
The Memory Problem Behind the Reading Problem
To understand why this matters, you need to know a little bit about something called orthographic mapping. It’s the process by which your brain permanently stores a written word for instant, effortless retrieval. When orthographic mapping works, you don’t decode a word every single time you see it. You just know it.
Reading Rockets explains it well: orthographic mapping connects the letters on the page to the sounds of the word, and then those sounds get linked to the word’s meaning. That meaning connection is the anchor. It’s how the word gets stored somewhere your brain can find it automatically.
For kids with dyslexia, this mapping process is harder. Their brains have more difficulty forming and retrieving those stored connections. And when a word has no concrete meaning to serve as an anchor, the problem gets even worse.
As reading expert Abigail Marshall of Davis Dyslexia Association International explains in her writing on orthographic mapping, dyslexic readers often stumble over short, familiar function words because the meaning hasn’t been “mentally imprinted.” The word might be fully decodable, every letter accounted for, but without that meaning anchor, it doesn’t stay stored for automatic retrieval. So a child can read a word correctly on one line and substitute something else on the very next.
That substitution isn’t carelessness. It’s a memory retrieval issue. Their brain reaches for the word and comes up empty, so it grabs whatever is close enough.
Why Sight Word Drills Often Aren’t Enough
Here’s where a lot of well-meaning classroom practice falls short. Standard sight word instruction typically involves flashcards, word lists, and repetition. You see the word, you say the word, you move on. For concrete words, this can work reasonably well because meaning reinforces memory. But for abstract function words, drilling the word in isolation doesn’t give the brain anything to hold onto.
A newer approach from Reading Rockets emphasizes teaching high-frequency words through their sound-letter connections rather than pure memorization, because mapping sounds to letters is actually what makes words stick. But even that isn’t the full picture for function words, because the meaning layer is still missing.
This is why researchers and structured literacy specialists increasingly argue that function words need sentence-level instruction, not just word-level repetition. The word “of” needs to be understood in context, in sentences, in multiple examples that show the reader: here is what this word does, here is how it changes the sentence, here is what would be different if it weren’t there.
What This Means for How Kids Get Instruction
If your child is in a reading intervention program, it’s worth asking specifically about how function words are being addressed. Not all programs approach this the same way.
What the research points toward is instruction that includes three things working together.
- Phonological grounding: making sure the child can hear and connect the sounds in the word, even when the word is short.
- Meaning-focused, context-rich exposure: showing the word functioning in real sentences, not just on a flashcard.
- Sentence-level practice: building and manipulating sentences that demonstrate what the word actually does.
David Kilpatrick, whose research on orthographic mapping has been foundational in the structured literacy world, notes that phonological awareness is the key driver of whether words get stored as automatic sight words. But for abstract function words, that phonological connection alone isn’t enough. Meaning has to be part of the picture, and for words that carry meaning only through context, context is where instruction has to live.
What You Can Do Right Now
At home, you probably can’t overhaul how your child’s reading instruction is structured. But you can pay close attention to patterns, and that attention is genuinely valuable.
Start keeping a list. The next time your child reads aloud, jot down every function word they miss, substitute, or skip. Not to stress about it, just to notice which specific words keep showing up. Is it always “of”? Do they consistently swap “by” for “to”? Do they drop “the” at the beginning of phrases?
That list is data. Share it with their reading specialist or intervention teacher. Ask: “What explicit instruction are we doing around these specific words? Are we working on them in context, in full sentences?” If the answer is just flashcard drills, that’s useful information for the conversation.
You can also try something simple at home. Pick one function word your child struggles with, say “of.” Read aloud together and every time you hit “of,” pause and name it. “There it is. What does ‘of’ do here? It’s connecting ‘bowl’ and ‘soup.’ It tells us what’s in the bowl.” That kind of narration is low-pressure and surprisingly effective at helping meaning stick.
The goal isn’t for your child to define “of” on a quiz. The goal is for their brain to build enough context around the word that it finally gets mapped and stored. That takes repetition, but repetition with meaning, not repetition in isolation.
It’s Not the Word. It’s the Anchor.
Your kid isn’t struggling with “the” because they’re not trying or not paying attention. They’re struggling because their brain processes abstract, contextual meaning differently, and standard reading instruction doesn’t always account for that.
Understanding the why changes how you advocate for your child. You’re not asking for something extra or unreasonable when you ask for sentence-level, meaning-rich instruction around function words. You’re asking for what the research says actually works.
Action step: This week, sit down for one read-aloud session and keep a notepad nearby. Write down every function word your child stumbles on or skips. By the end of the week, bring that list to their reading specialist and ask how those specific words are being addressed in their current instruction.
About Decoding Mom
Decoding Mom is written by a mom of a bright kid with ADHD and mild dyslexia. After too many late-night research binges trying to make phonics fun, she started this site to translate the science of reading, IEPs, and special-ed assessments for parents figuring it out the hard way. Honest, parent-first, no fluff. More about her here →
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