One language, two number-word systems and many problems: numerical cognition in the Czech language.

Researchers watched Czech first-graders write two-digit numbers after hearing them aloud.

Kids heard either the everyday inverted form (five-and-twenty) or the clear non-inverted form (twenty-five).

The team counted how many times children flipped the digits when writing.

Children made far more reversal errors after hearing the inverted form.

The tricky word order alone caused the mistakes; the kids understood the quantities.

Linguistic structure, not math sense, drove the transcoding slips.

Schwenk et al. (2017) pooled many studies and found slow symbolic comparison marks math trouble. Their meta-analysis quietly includes inversion errors like these, so the Czech result is one clear example of that broader pattern.

Peters et al. (2020) seems to disagree: they link dyscalculia to weak spatial skills, not to number magnitude problems. The two papers clash only on the surface—S et al. show language trips kids who actually know magnitudes, while Lien shows some kids fail math for spatial, not linguistic, reasons. Different routes, same outcome: wrong answers.

Defever et al. (2013) used mixed-notation matching and also blamed access, not core number sense. Both studies say the symbol-quantity link breaks down under extra load, whether from notation mix or inverted wording.

When you teach bilingual or language-learner students, skip the inverted number words. Say twenty-five first, show the digits 2 and 5, then mention the old form if needed. One small wording tweak can cut reversal errors on the spot.