What are key techniques to show I have understood an source in an exam?

Firstly, read the source thoroughly. It sounds obvious but reading the sources two or three times will help improve your understanding of its content and its arguments. Secondly, identify the individual strands of the main argument: every source will have one over-arching main point, but this will be bolstered by evidence which supports it, and identifying these also will show that you understand both the main argument and how the author has made their case that it is true. Thirdly, identify and include key quotes which show how you justify your conclusions. Remember that unlike science, English has no definitive right or wrong answer - as long as you argue your point well and it is relevant and justifiable, there is no reason why it should not be worth marks. Lastly, bring in relevant outside knowledge if you have any. Linking the article or making astute comments on its content show that you have not only understood the text, but have actually engaged with it. Those sorts of details can really wow a marker when they are faced with a pile of identical answers!