The natural order hypothesis is the idea that children learning their first language acquire grammatical structures in a pre-determined, 'natural' order, and that some are acquired earlier than others. This idea has been extended to account for second language acquisition in Krashen's theory of language acquisition.
According to the natural order hypothesis, learners acquire the grammatical morpheme -ing before the morpheme third person -s.
In the classroom
One possible implication of this hypothesis is that teaching language through a traditional structural syllabus may not necessarily help them to acquire the language they need. Attempts to get the learners to produce structures before they are ready to do so may fail.