How do I answer a question asking me to comment on what I can learn from a source?

There are two main things to look at in a source: 1) the blurb & 2) the text or image itself.
When analysing the blurb look for a few key questions such as who? when? and where? For example, if the source was a painting of an operation of an American hospital in 1875 you can extrapolate a lot of information from this, firstly that this is an example of western medicine as the surgery is taking place in America and furthermore that this is only in relation to late 19th-century surgery. So only make sure you are not using evidence from the middle ages for example. Most of the information will be held in the actual source itself, for image sources look at the content of the image and make an inference from it. For example, if it is a photograph of a Nazi SS rally in the 1930s and there are lots of Nazi flags held up by the Nazis then you can infer that uniformity and symbolism were key to the propaganda of the party.
When answering the question make sure you actually answer what the question asks, not just what you know from the source. If it is a question that asks you to compare two sources to show change or continuity over time, then do so & make sure when you make an inference back it up with supporting detail from the source and your own knowledge.