Are the Four Noble Truths simple to understand?
The Four Noble Truths are not simple to understand and requires investigation and contemplation over a lifetime in order to penetrate their meaning and to realize the Dhamma.
How extensive is dukkha? How do we deny or run away from experiencing dukkha? How is dukkha concealed by postures?
What is craving? Who or what is generating the attitude of craving? Who or what is experiencing craving? If we remove the instigator of craving then will craving vanish?
What is it that remains when suffering and craving is not present? Where is the instigator of craving and can it be located when craving is eliminated?
How does the Noble Eightfold Path act on the mind which allows suffering and craving to be removed? How deeply and refine can the practice of the Noble Eightfold Path be taken? How does each path factor change and purify the mind? What is changing in the mind with the practice?
These and many more questions are to be investigated in order to penetrate the Four Noble Truths. The answers to the questions lies in the practice.