We know that past is the set of events that has already happened, and future is the set of events that is yet to happen, and hence unknown.
Present is merely a transition from past to future.
Ok, but what we think to be present, is it really present? I mean, look at the night sky, you see thousands of stars. You think you are seeing them in the present. But that star that is millions of light years away may not really exist at this moment! Why? Because what you see is the light that left the star that many million years ago.
So the stars we see in the night sky belong to a snapshot from the past, not present. And each of those star images belongs to a different time in the past. If there are two stars visible next to each other in the night sky, and one is say 50 light years away from earth and the other 500 light years away, then even though we see them both at the same time, what we actually see are the images that are 450 years apart! The first star image is 50 years old, and the second one is 500 years old!
Even the sun we look at is 8 minutes OLD.
To see how the sun looks now, we have to wait for another 8 minutes because light takes 8 minutes to get to earth from the Sun. The moon image we see is 1.3 seconds old, light takes 1.3 seconds to reach us from moon.
Since light takes some time to travel, no matter how fast it travels, all the images we see even in our immediate neighborhood, are all a tiny fraction of a second old. And then, our brain also takes its time to process what we see and create a picture out of it. The light signals hit our cone cells in the eye, then the signals are filtered and travel along the optic nerve to our visual cortex and then the brain further processes the image to create its own picture of the world. All this takes some time.
So what we think to be present, is not the present, but a time delayed snapshot of the past, and how much in the past depends on what are the things that we are looking at.
Added to this is the fact that brain is a prediction making machine. If you are trying to hit a mosquito, the brain is trying to predict the next position of the mosquito in the future so that you can accurately target it. Flies move in an unpredictable zigzag pattern, so brain finds it difficult to predict the next fly position.
When you are traveling in the road, when the spoon in your hand is about to fall, when you are listening to music, etc – always the brain is trying to predict what happens next.
In other words, there is no present that we really experience. What the brain computes to be present is actually a thing of the past, and what brain computes us to expect is actually a thing in the future!