It could be anything from verbs formation to relationship between sentences, word classes, etc. The structure of the English language can be analyzed in many ways. For example, you can study classes of words (parts of speech), meanings of words (semantics), with or without considering changes of meaning, how words are organised in relation to each other (syntax), how words are formed (morphology), the sounds of words (phonology) and how written forms represent these (lexicography).
For more on this see the Grammar Guide in the
ICALwiki 
Major Language Structures 
Linear Mode
