Study your subject before you start writing. Regardless of your writing skills and your ability to compile a 400-word text from scratch, the reader will understand whether you are aware of the subject or you just improvised.
Present the information as a list of features, or if the content is contradictory, use the compare and contrast method. On the latter, the essay will consist of four paragraphs, while for the list presentation, do not include more that five paragraphs.
Prepare the structure of your essay. Plan which pieces of information to use in each paragraph and which irrelevant topics to omit. This will save you from erasing parts of paragraphs and hastily patching others, which, during school test, can prove disastrous -- especially when using pens.
Express clearly the main idea of your essay, as well as background information about the topic, in the introduction. The introductory paragraph must not be larger that 100 words, or 80, if you write a five-paragraph essay.
Clarify the subject of each paragraph of the main part. When readers have finished reading the first sentence, they must know what the paragraph is about. The paragraph's last sentence should prepare for your arguments in the following part.
Conclude the essay by summarizing its contents, or suggesting solutions for the presented problem. Never omit the conclusion, even if you have much more information to give than 400 words allow.