One to four mark questions will start with command words such as 'describe' or 'explain'. Some command words are easy to understand such as:
The command words 'describe' and 'explain' can be confusing. If you are asked to describe a graph, you will be expected to write about its overall shape, whether it is linear or curved, the slope of gradients etc. If you are asked to explain why a pattern or trend is seen in a graph, you will be expected to use your science knowledge, not just say what you see (which is a description), eg the graph shows a steep linear increase for the first three hours because…
Explain how and why questions often have the word 'because' in their answer. Describe questions don't.
The number of marks per question part is given in this form '[2 marks]'. It is essential that you give two different answers if a question is worth two marks. Sometimes you can gain a second mark by giving the units in a calculation or stating specific data points, eg during the first two years the number of lions deceased by seven.
You will be expected to write in more depth for three and four mark questions. They might ask you about a process such as the carbon cycle or the method you would use in an experiment.
Edexcel questions courtesy of Pearson Education Ltd.
A student wanted to estimate the number of earthworms in a field using a quadrat.
The student placed the quadrats at random on the surface of the area being sampled and then watered the area with a very dilute solution of mustard.
This causes the earthworms to come to the surface to be counted.
Give a reason why the quadrats were placed at random. [1 mark]
To obtain a representative sample of the field. 
Give two abiotic factors that could affect the distribution and size of daisies growing in this field. [2 marks]
Marks will be given by using any of the following:
A quadrat sample was taken in an area where there were many overhanging trees. Explain how these trees may have affected the distribution of daisy plants growing in this area. [2 marks]
Full marks will be awarded for including the following points:
People who grow vegetables in their garden use compost and manure to help maximise the yield. Explain the similarities and differences between manure and compost. [3 marks]
Both are made by decomposition.  Manure is made from solid animal waste (faeces). Compost is made from dead plant material.  Both fertilise the soil by providing key minerals and nutrients. 
Water pollution can be caused by an increase in nitrates and phosphates. Explain the problems associated with an increase of nitrates and phosphates levels in a lake. [3 marks]
An explanation linking three of the following:
Figure 1 shows a woodland food chain.
Tree → Caterpillars → Sparrow Hawk
Describe what happens if caterpillars are killed by an insecticide. [2 mark]
The sparrow hawk will have less food and may die or eat other organisms. 
The tree will have fewer leaves eaten by caterpillars, which may be eaten by other organisms.