The 600 words in each book of this series along with the additional target words presented in the appendices included in the first three books of the series are the most useful words in English. They were found by analysis of a collection of English course books from various levels in the primary, secondary and tertiary school systems. The words included in this series were chosen because they occurred many times in different levels of these materials. Because of the way that they were chosen, these words have the following characteristics:

1. They are useful in both spoken and written English. No matter what English course you are studying, the words in these books will be of value to you.
2. Each word in these books is a high-frequency word. This means that the effort in learning the words is well repaid by the number of times learners have a chance to encounter or use them.
3. These books as a whole cover a large proportion of the words in any spoken or written text. They cover at least 80% of the words in newspapers and academic texts, and at least 90% of the words in novels. They also cover at least 90% of the words in conversation.

The activities in "4000 Essential English Words" are specially designed to make use of important learning conditions. Firstly, the words are introduced using sentence definitions and an example sentence. The activities that follow in the units encourage learners to recall the meanings and forms of the words. Some activities also make the learners think about the meaning of the words in the context of a sentence—a sentence different from the sentences that occurred in the introduction of the words. Moreover, each unit ends with a story containing the target words. While reading the story, the learners have to recall the meanings of the words and suit them to the context of the story. Such activities help learners develop a better understanding of a common meaning for a given word which fits the different uses.

Illustrations for each target word are provided to help learners visualize the word as it is being used in the example sentence. These word/image associations aim to help students grasp the meaning of the word as well as recall the word later. It should be noted that words have more than one grammatical category. However, this series focuses on the word’s most common form. This is mentioned to remind learners that just because a word is labeled and utilized as a noun in this series does not mean that it can never be used in another form such as an adjective. This series has simply focused on the word in the form that it is most likely to be expressed.