You can see the original string in context (if this has been made available by the client) under the Context tab. Depending on the source file type, the context might update to reflect the target language translations as you translate the strings. If this is the case, you will see an additional menu to toggle whether you want to view the context in the source or target language.

If context is enabled for your project, you can click the cogwheel icon to open it in a new window and change how you want the view to highlight the strings that you are currently translating. If there is no context available for your project, you can collapse this view by clicking the double arrow button in its bottom right corner.