Research shows that there are systematic and predictable developmental stages, or sequences, of second language acquisition.
It is important to emphasize that developmental stages are not liked “closed rooms”. Learners do not leave one behind when they enter another. It is common that learners produce sentences typical of several different stages.
It is better to think of a stage as being characterized by the “emergence” and “increasing frequency” of a particular form rather than by the disappearance of an earliest one.
Even for a more advanced learner, conditions of stress or complexity in a communicative interaction can cause the learner to ‘slip back’ to an earlier stage.