Suppose I have 2 perpendicular sides of triangle, which are only known to me. Can anyone tell me the length of the 3rd side? No! But still the 3rd side exists. Any helper at best can tell me to use the "Pythagoras method". Similarly, the amount of your knowledge (GyAna) and type of actions (Karma) are only known to you. No one can objectively tell your Dharma, but you. Yes of course there can be methods, which can be subscribed from the eternal truths derived out of relevant scriptures.
Though not entirely, Dharma will differ person to person more or less, because all individual are different. Neither they have exactly same knowledge, nor do they act similarly.
Following Swa Dharma = SAtvika action(s)
Don't limit SAtvika with a very calm & non-violent person alone.
One easy way to decide right/wrong is to keeping your self in others' shoes (situation). Will I like if done with me?
"'How' can we decide what is dharma, in a 'particular' situation.
'How' -- Natural Selection.
'particular' -- Ofcourse, no single answer. Quite possible that, if you and me are put in a "particular" situation, where we may act differently, yet both act perfectly in Dharma.
"Natural Selection" is the way of SAtvika life. It's neither "Selfish Selection" (RAjasika) nor "Impulsive Selection" (TAmasika).
[BG 18.47] — Defective (faulty) Swa Dharma is better than well performed Para Dharma; Naturally prescribed performance of Karma never achieves sinful reactions.
[BG 18.48] — O son of Kunti, trivial(natural) Karma should never be given up even though (they are) defective; All initiations (of Karma) are defective, like how smoke covers the fire.
Above 2 verses say it all. There are always faults (findings) in all the actions. But just because the actions are faulty, one should not abandon them; If really abandoned then, such wrongful sacrifice is RAjasika [BG 18.8].
- Lot of bacteria are killed when we boil the food, but then we don't
play with our health
to save them
- The environment is polluted when vehicles run on the road; But in
today's time, the economies will collapse if people stop commuting
the way they do today;
So we prioritize the actions depending on situations. It doesn't mean that we don't have any commitment towards environment & correcting ourselves. But the corrections have to be "natural" rather than "abrupt".
"Natural Selection" isn't guaranteed to be harmless, it's just the path of "possible least harm", which can miserably fail as well. Still it's preferred.
Being natural allows us to not to be conscious about our 'false' self.
"Dharma is eternal truth, I am sure it is unchangeable"
Correct. Dharma is eternal truth and it defines separately for different entities in form of Swa Dharma. The Swa Dharma may not change so easily.
"so what is dharma for Rama and Krishna is dharma for me."
Nope. To me it's naive to relate others' Dharma with own's. The verses are already mentioned above.
To win a war, RAma probably would never create a "false hoax" in the active battlefield and ask any of his commanders to say lie to sustain such cheating. But Krishna did with Drona and few others, despite Krishna himself regarded the truth as supreme.
For Rama "truth" was priority while deciding Dharma, for Krishna "justice" was priority while deciding Dharma. Both acted well in Swa Dharma with clear contradictions.
"someone with intent of destroying asks about my address, what should I do?"
It depends on who you are. There is a nice quote from a very good answer from @sv:
Truth may be unutterable, and even falsehood may be utterable where
falsehood would become truth and truth would become falsehood.
In a situation of peril to life and in marriage, falsehood becomes
utterable. In a situation involving the loss of one's entire property,
falsehood becomes utterable.
He is a fool that practices truth without knowing the difference
between truth and falsehood.
Suppose during your upbringing, if you always learn to prioritize ...
- ... Truth over everything then that becomes "nature" of yours. Hence in above situation, you should say the truth and let the person
destroy the property. e.g. RAjA Harishchandra who let go all his wealth and family to be truthful
- ... Protection over everything then, truth becomes secondary. In such case you must naturally misguide the person by any means or
falsehood. e.g. Unknown young martyr from Kashmir who misguided
Pakistani Army in 1947 to disallow occupation.
To me, both are good examples of following Dharma properly.
"Manusmriti Dictates that bramhachari should not communicate with any
You have answered in your question already. Things change with time.
Certain truths are eternal (e.g. maths) and certain truths are local to time (e.g. way of commutation). 1 + 1 = 2 will always be as it is, irrespective of when it was discovered. However, people have stopped commuting on horses for any general travelling.
Finally, all the actions are not in anyway our control. They are done by God. e.g. Not fighting the war was not a choice for Arjuna. It was a temporary repulsion before the battle started. He was anyway destined to fight:
[18.59] — Taking shelter of (false) ego, if you believe that "I won't fight", then it's incorrect decision. Your nature will engage you (in war).
[18.60] — O son of Kunti, involuntarily you will be bind to do even that Karma born out of the nature, which you don't desire due to illusion.
So despite any advise, you will do what is destined for you!