English vocabulary is of two very different kinds*. It consists firstly of the predominantly one-syllable Anglo-Saxon words like fire, climb, ask. And secondly there are the multi-syllable words like conflagration, comprehension, interrogate which came into the language from Latin and Greek after the Norman invasion in 1066.
Although the short Anglo-Saxon words constitute only about 10% of total English vocabulary, they are the mainstay of everyday conversational English - in fact they make up almost all of the 100 most commonly-used words in the English language. The longer French, Greek or Latin-origin words are found in formal settings such as education and government.
In general**, the short words of Anglo-Saxon origin are easy to learn and use. Since these words are also the most common ones in the language, it is no surprise that many ESL learners pick them up quickly and soon become proficient in conversational English. The other words (of French, Latin or Greek origin, which comprise about 90% of the language) are harder to learn, often because they convey abstract concepts. However they are prerequisite to the comprehension of academic texts. Reading comprehension, in turn, is a prerequisite of academic success.
This interdependence of vocabulary knowledge, reading ability and academic achievement is summarized by Folse (2004):
"Nonnative speakers must have good reading skills if they expect to have any chance of academic success. Numerous researchers have shown the relationship between vocabulary knowledge and reading ability."It is evident, therefore, that students who want to do well in school will have to build a large vocabulary.
In summary, learning vocabulary is the most important thing that ESL students should focus on in order to achieve academic success - over and above completing the regular class work. Teachers who follow the above advice will be making a significant contribution, not only to the student's English vocabulary development but also to his or her ability to do well in their subject.
Folse, K. 2004. Vocabulary Myths: Applying second language research to classroom teaching. University of Michigan Press.