Your normal form would modify the damage assuming that instance of damage has not already been modified the same way.
Resistance/Vulnerability can only be applied to a given instance of damage once.
This claim is ironically supported by a response by Jeremy Crawford misunderstanding a question on Twitter about Warding Bond but it does reword PHB p197 to make a touch more sense.
Resistance is applied only once to any instance of damage. See the
Player's Handbook, page 197.
This is a blanket statement that is clarifying a misunderstanding that some seem to have. Warding Bond, and its ilk, break one instance of damage into two whereas the OP's question would be a single instance of damage affecting the same creature (unless you think that polymorphing makes you a separate creature, which it doesn't).
Multiple instances of resistance or vulnerability that affect the same
damage type count as only one instance. For example, if a creature has
resistance to fire damage as well as resistance to all nonmagical
damage, the damage of a nonmagical fire is reduced by half against the
creature, not reduced by three-quarters.
If you apply resistance after it has already been applied you are essentially instigating that last line of trying to reduce it by three-quarters or lower since it will be even less by the time it gets to your standard form.
The Difference in the linked questions.
To help alleviate the conflation between the other questions linked as they don't work quite the same way I offer the following:
In the cases of Shield Guardian/Warding Bond effects the benefactor of the effect (the wearer of the amulet in this case) is hit with an arrow for 40 damage. The wearer never takes that damage as it is split prior to application so that 20 affects him and 20 affects the construct then resistances and immunities are applied. This effectively makes them a separate instance of damage for each creature involved and thus each can benefit independently from resistances/immunities/vulnerabilities.
Damage A is rolled and immediately split into Damage A1 and A2 before application.
The issue at hand.
This issue is not splitting the damage between two creatures before application it is actually applying a single instance of damage to a single creature. Resistance would be applied once and only once to that instance of damage and thus in your example:
- Assumed form doesn't have resistance and standard form does which is your main question... You take 40 get reduced to 0 then the remaining 30 has resistance applied so that you take 15 as no resistances have been applied to that instance of damage, yet.
- Assumed form has resistance... incoming damage has resistance applied and is halved. So you take 20, this reduces your assumed form to 0 and the remaining 10 is applied to your standard form.
- Assumed and Standard form have resistance... incoming instance of damage is halved by application of resistance. You take 20 damage which reduces you to 0 you fall out of your assumed form and since this instance of damage has already been affected by resistance it cannot further be affected by it therefore you take the remaining 10 to your standard form.
- Assumed form has vulnerability and standard form has resistance... incoming damage is doubled to 80, you take 10 and drop to 0 the remaining 70 damage is reduced to 35.
- Assumed and Standard form have vulnerability... incoming damage is doubled to 80, you take 10 and drop to 0 the remaining 70 damage is take by your standard form, because that instance of damage was already modified by vulnerability. (i.e. it can't be subject to vulnerability twice before application to the creature)