Corpora can give us valuable information about grammar as well as vocabulary. For example, a corpus can tell us how common structures are distributed. As we have said before, we cannot rely on our intuition to tell us what items are used more frequently than others. When we are thinking about grammar, rather than lexical items, we can still ask the same sorts of questions about frequency. For example, Mindt (2000:224) identifies four uses of the present perfect in English
- The indefinite past
- Past continuing into present
- The recent past
- Completed action at an unspecified time in the past
Corpus research has shown that 80% of the occurrences of the present perfect fall into the first category of use (the indefinite past). Of the remaining occurrences, 95% are examples of the past continuing into the present. The last two meanings of the tense are therefore extremely rare. Hunston (2002) argues that a survey of English teaching materials demonstrates that the less frequent forms are presented as prototypical examples of the tense use, when in fact they are not.
By comparing spoken and written corpora we can compare the grammar of speech and writing. Once we start observing how grammar works in both these modes of communication, we can see that, although there are many similarities, there are also key differences. Underlying this is the fact that, as McCarthy (1998:76) points out, 'we should never assume that if a grammar is constructed for written texts, it is equally valid for spoken texts'. For example, analysis of spoken corpora shows us that ellipsis (omission) of things like subject pronouns, articles and auxiliary verbs is common. Examples of this kind of ellipsis can be found in Discourse for teaching purposes by Dimitrios Thanasoulas. Corpora are also able to show us that such phenomena are not isolated slips of the tongue, and that they are not the 'lazy' speech of the younger generation.
McCarthy and Hughes (1998) argue that there is ample corpus evidence that second language teaching and materials design should be informed by discourse grammars. They conclude by saying that 'a practice-driven view of grammar teaching should be fostered, with the teacher being empowered to take an active part in, and an informed view of, what features to present and how best to present them'.