Most 13- and 14-year-olds have heard of nouns and verbs but do not really know what they are, says the study from Southampton University, part of a five-year research project funded by the Economic and Social Research Council.
They are getting mixed messages about grammar, with modern language teachers emphasising its formal teaching while English teachers encourage children to learn grammar by using the language.
Professor Martin Hughes, the research programme's co-ordinator, said: "It's ironic. If you want to learn about your own language you need to learn another."
Professor Christopher Brumfit and Dr Rosamond Mitchell, the study's directors, spent a year analysing dozens of lessons by seven teachers in three schools. They also assessed children to discover how much they knew about language.
While foreign language teachers taught grammar in a traditional way, concentrating on individual sentences and words, English teachers tended to teach about whole texts and were more interested in style and genre than grammar.
The study found that there was very little formal grammar teaching in English lessons. Teachers often do not know enough about language to teach it properly, they concluded.
Dr Mitchell said pupils learned basic definitions of nouns in primary school but that was not developed. We needed to be more systematic about language teaching and teachers needed to be told what pupils of a particular age should understand about grammar. "Most teachers do imaginative things about language but in a very patchy way."
Professor Hughes, of Exeter University, said: "Pupils' limited understanding of their own language is a serious cause for concern."
He said it highlighted the problem of the division of the national curriculum into individual subjects. "What children are learning in one subject may be related in no way to what they are learning in another."
Anne Barnes, general secretary of the National Association for the Teaching of English, said the research suffered from confusion about what the word "grammar" meant. "It means the ability to construct a sentence so that the meaning is clear. It can also be used as a system which provides a short cut to learning a foreign language. The two have very little in common.
"Of course children learn to give names to parts of sentences to learn a foreign language but that isn't something you particularly need when you are speaking and writing your own language."
Professor Hughes' research programme looked at the nationwide impact of government changes on the education of pupils aged five to 16.
Another study, from the Thomas Coram Research Unit in London, found that, over the past 10 years, the time spent by schools on the basics of English, maths and science, had changed little despite the introduction of the national curriculum.
Some experts have suggested that the nine-subject curriculum in primary schools has squeezed the basics.
Researchers found that thetime teachers spent hearing each child read every week had not changed at all since the mid-Eighties. Then and now, it was just eight minutes.
However, young children's school days have become more academic. There is more science and less art and craft than a decade ago. The report says the amount of time children spend reading is worryingly low: it is vital that schools involve parents to ensure that all children read at home as well as at school.Reuse content