I have to agree on the virtues of the P (program) mode. It’s in fact a manual mode on steroids.
I don’t know how it specifically works on Nicons , but I suppose it works like in any other brand. You start with a computed exposure but you may change any parameter, (shutter speed , aperture, ISO) and the camera resets the others accordingly. You may even use EC (exposure compensation) in order to push the exposure in your desired direction. Change the metering mode and you have a plethora of other possibilities. Lock exposure and recompose and you obtain another shot entirely. So, If used correctly and intelligently, P mode may be very creative and also fast, and it may be very useful even for experienced photographers. I won’t look down on it and use manual only because it’s a more “pro” mode. Yes there are situations when manual mode is preferable, or A / S for that matter, but P mode, even if it may be used like a P&S camera, is not less creative in a DSLR than the rest of the “pro” modes.
Viewed from a didactical perspective, I find it hard to decide, I’ve learned to shoot with analog cameras whit a paper table that was in every film box that listed the correct exposures for various situations, basically what JAWAD asked in his post.
JAWAD you camera has a light meter that shows you what to do. Read the manual and learn how to use it and you will be able to set your camera in manual mode for every situation.
You will learn from reading and experience what triplets of settings (shutter speed , aperture, ISO) to use,(it’s logical), but he light meter is always your starting point. In the P, A,S modes the camera decides for you based on the same metering, in M mode you set them yourself according to the effect you want.
How to know what to choose? Read about depth of field, motion blur, freezing motion, backlight , composition and so on. It will be gradually clearer.
Canon PowerShot S100
Canon 50D , SIGMA 10-20 f3.5 ,Canon EF 24-105 L IS USM, Canon EF 100/2.8 macro Canon EF 50/1.4 ,Canon EF 85 f1.8,Canon EF-S17-85 4-5.6 Old Tamron 28-300 inherited from my Canon Rebel G film camera