Living in the present
And another answer about Krishnamurti's notion of living in the present moment:
Your question touches on a complex topic which can be interpreted in many ways. To start with, it's important to clarify that the concept of “staying in the present moment” doesn't necessarily mean never thinking about the past or future. Rather, it's about not being unnecessarily preoccupied with them to the point where it detracts from our ability to experience and respond to the present.
On the subject of Jiddu Krishnamurti, his teachings are indeed quite radical. He emphasizes a form of awareness that's fully immersed in the present moment, not in the sense of never planning for the future or learning from the past, but in the sense of not being psychologically bound by them. This doesn't mean that we should abandon all thought and planning, but rather that we should recognize when our thought processes are serving us and when they're leading us into needless suffering.
Now, when it comes to the question of how immersing in the present moment can bring about right action, it's useful to consider the nature of attention. Our attention is like a spotlight. When we're absorbed in thoughts about the past or the future, that spotlight is directed away from what's happening right now. By bringing our attention back to the present moment, we can perceive the current situation more clearly and respond to it more effectively.
As for the nature of time, this is a vast philosophical topic. In this context, though, one possible interpretation is that the past and the future exist only in our thoughts. The past is a memory, and the future is an expectation. The only thing we can directly experience is the present moment.
Finally, regarding the psychological perspective, rumination (persistent and repetitive thinking about the past) and worry (persistent and repetitive thinking about the future) are both associated with a variety of psychological difficulties, including depression and anxiety. Practices like mindfulness, which emphasize attention to the present moment, have been found to reduce rumination and worry, and to have beneficial effects on psychological wellbeing.