According to Manfred Pieneman and his associates, some linguistic structures develop along a predictable developmental path and this refers to developmental features.
This proposal gives information about the development of the learner language. Also, there are some variational features which depend on many factors such as motivation and aptitude. These certain aspects of language can be taught at any time.
As a conclusion, it is suggested that while some features of the language can be taught successfully at various points in the learners’ development, other features develop according to the learners’ internal schedule.