The way I look at it, to augument what is already stated as I read these other comments as saying the same thing....
Water, is always wet when it presents as water. This is not a perfect analogy but may express this a bit.
Solid it is ice not water. Gaseous it is mist or fog not really water, though composed of water.
Water is not present everywhere nor can we assume if the universe degenerates into a supermassive black hole, it will always exists.
But when it exists it is wet always.
So the dharma is the understanding...water when it presents is wet always. The dharma that true thing is always true.
Suchly is the dharma eternal.The truths are always true and permenant but the circumstances surrounding our particular display(where we find ourselves) will not always be beneficial to all productions.
Regardless if we are there to witness it or if water is actually present....it is always wet.
It is a truth.
So are truths negotiable in that they change with circumstance.....well not really.
They change only in response to the cirumstance....we can't say water is everywhere nor that it cannot be ice or fog or other things or not exist at all..but when it presents as we perceive it, water is always wet.
There is always a compounding with truth as they are observations about objects. But a compounding things does not infer always a changing thing in this sense. Quality of thing tends not to change with circumstance. Whether we are animal plant human or on earth mars or some other place water will always when it presents be wet.
The understanding of things they say some that produces nirvana. It is not that we are literally in a different place(all places are the same) understanding things as they are produces the place of nirvana as opposed to samsara.
We are conginitive things that understand. Our awarenesses speak to that fact. Suchly in a practical sense the more we understand the more we are at peace with things of any sort. REally our likes and dislikes are arbitrary and in relationship to a false construct a self.
The more we understand about things of all sorts and types(such as the me) the more we may be at peace or in a place some call nirvana.
AS cogntive things our reality(some say in a actual sense some not) how we perceive is how we order or create it.
A self fullfilling prophesy of sorts with me comes samsara and sorrow. With understanding(dharma) comes the production of nirvana.
A real nirvana** a heaven of sorts would be a discrimination of sorts..produceable but with aspect of aversion and like.
As such a unreal nirvana would be a place of mind that finds all discrimination finding all things as we are with no real source for aversion nor attachement, likeing or disliking.
There are different views on this thing so opinions vary. This may explain one way of looking at it. The first part coincides with the opinions already versed but adds a differing context. The second part is particular view which others may hold or not,I don't know.
Maybe that helps maybe not.
**real or unreal in this context refers to the variance of envisioning of this thing not the ultimate reality nor unreality of things of this sort. How they actually subside,in what mileau, that is another issue.
So can we forget truths....sure. a bit. WE have retentive aspect to our consciousness always, as part of cognitive function always.
Once found a thing is consciously known but habitual inclination and thuly the unconscious me tends to erode it.
With significant habitation complete intigration and full understanding such things may not be eroded.
The erosion, any that may occur, occurs on the basis of the me, working against or putting things in the context of me and other.
Completely understood, integration may be said the me to be dissolved.
So as me dissolved, the dissolution or erosion being of construct of me stops,
So without agent of dissolution, but with retentive aspect, consciousness retains that which is known of as true..that water is always wet and does not forget it or find it overlayed by other things.
So when nirvane is assumed...yes it is final as no cause exists to serve as dissolving agent. Me it is that essentially does the forgetting otherwise our conscsiouness is perfect in its retention. So the understanding the dharma is permenant in both aspects..always true and when underestood totally by us as cognitive things always present in consciousness and not able to be disintigrated. Of course us being under the full influence of me may forget things we learn today tomorrow. But with enough time and effort insight and study and habitation of a sort, me is dissolved no agent exists for retentive disruption...we remain with consciousness and perfect retentive aspect, not disrupted by me influcence.
I contend we do not necessarily always exist but others hold other view and that is another matter aside the topic a bit.
AS I read these things.
"This order considers that progress can be achieved more rapidly during a single month of self-transformation through terrifying conditions in rough terrain and in "the abode of harmful forces" than through meditating for a period of three years in towns and monasteries"....Takpo Tashi Namgyal.