The verb - When you concentrate you direct all your efforts towards a particular activity, subject or problem.
For example: You need to concentrate harder when you listen to something in another language.
The adjective - If something is concentrated it means it has had some liquid removed.
For example: I prefer freshly squeezed orange juice to concentrated.
!Note The simple past of "to concentrate" is "concentrated" this is where the confusion may arise.
For example: She concentrated very hard in the exam.