F80 vs F81

Both are specific developmental disorders sharing the same underlying logic (early onset, steady course, deficit out of proportion to general ability, not explained by opportunity or a sensory/neurological condition), and a child with an early spoken-language difficulty is at higher likelihood of later scholastic-skill difficulty, so the two can appear together or be confused for one another.

• F80 is defined by a specific impairment in the development of spoken language — expressive language, receptive language, or articulation — from early childhood, before formal schooling typically begins.
• F81 is defined by a specific impairment in an academically-taught skill (reading, spelling, or arithmetic) that emerges once formal instruction in that skill has begun, with normal early spoken-language milestones.
• A child with normal early spoken language who later struggles specifically with reading or spelling once schooling begins favours F81 over F80.
• A child with an established early spoken-language delay or disorder, assessed before school age, favours F80, whether or not scholastic difficulties are anticipated later.

What weakens F80: A clean early language-developmental history weakens F80 regardless of later scholastic difficulty; a clean scholastic-skill history despite an earlier language history weakens F81.

Both require onset in the relevant developmental window for that skill — F80 in the pre-school period for spoken language, F81 once formal instruction in the specific scholastic skill has begun — and a steady course rather than an acquired loss.

A single skill domain being affected does not by itself resolve which category applies; the specific domain affected (spoken language versus an academically-taught skill) is what decides it, and both must independently satisfy the shared exclusion criteria in the specific-developmental-disorder capsule (opportunity, general ability, sensory/neurological status).

Whether the primary, presenting difficulty is in spoken language before school age, in a specific academically-taught skill once schooling has begun, or genuinely both, each independently assessed against its own exclusion criteria.

Assuming an early language delay automatically predicts, or should be classified together with, a later scholastic-skill difficulty, rather than assessing each skill domain against its own criteria and history.