Thai marks quantity with a separate counting word. Say three cats in Thai and the word for cat itself never changes shape. English does the opposite: the noun changes, and cat becomes cats. For a Thai child that is a brand new idea, not a spelling rule. Once you see that, it stops being a mystery why the s keeps going missing after you've taught it. In the child's head, the number was already announced by the numeral in front.
Three Steps Are Enough for Lower Primary
- Step 1, just add s: cat to cats, dog to dogs, book to books. Drill it until it comes out automatically with a number attached: two cats, three dogs.
- Step 2, add es when the word ends in a hissing sound, s, x, ch or sh: box to boxes, watch to watches. Tell the class the e is there because without it the word is unsayable. Ask them to try boxs out loud and watch their tongues tie themselves in knots. A reason like that sticks far better than a dry rule.
- Step 3, the rebels: man to men, child to children, foot to feet, fish to fish. Teach these as a short list to learn by heart and don't go hunting for a rule that isn't there.
Make It Automatic by Counting Real Things
The activity that works best is counting things in the room in full sentences: I see three windows. I see ten desks. It forces every child to build the number and the plural noun themselves, live, every single time. The worksheets that suit this are the picture ones where children write the quantity along with the noun, followed by the type that asks them to circle the mistake in a sentence like two cat. Spotting somebody else's error is practice children never object to.