What does my teacher mean when he says my essay doesn't link?

Unlike in GCSE History, at A Level the arguments you make in your essay need to link together to then provide an answer to the question. Let's take the question "why did war break out in North America in 1754?" Now because this is a 12 mark question and you are being asked "why" war broke out in 1754, you need to provide an answer and should look to provide three clear, distinct reasons for this. One trick to make sure you get the different distinct factors is to try and categorise your ideas into three different categories depending on their impact: Underlying/long term factors, developed short term factors and the 'trigger' factor. In answer to this question we can come up with three distinct factors: longstanding tensions between Britain and France, disputes over land in the Ohio Valley between the two states and finally the murder of Jumonville by Washington's soldiers. Now you have three factors, you can create the link. The easiest place to do this in the first place is in the introduction. Here, very little has to be said other than outlining the three factors you're about to write about. In this case, you could say something like "Underlying tensions between England and France since the colonisation of America put pressure on either side to appear strong. However, these divisions were highlighted in particular in the dispute over land in the Ohio Valley. With strong tensions, war was eventually trigged by the murder of French diplomat Jumonville, giving France a viable reason to attack". After then explaining the three points in the essay, the conclusion can take much the same form of the introduction, only you can now add in the arguments you have brought in throughout your answer. And there you go, a simple way to structure a 12 mark answer!