Interlanguage Development

the process of learning a second or foreign language.

Stages of Interlanguage Development
Although they are not necessarily distinct, the process can be divided into three stages:

1.  Lexicalization : developing a vocabulary

2.  Grammaticalization : learning and applying grammatical rules

3.  Sophistication : developing a more nuanced understanding of the target language and learning to express meaning more precisely

Bruntt, Karen and Bryanne, Ulla. Handbook for Grammar Detectives, page 25.