Hello again!
As you said(& thanks for the topics),I've broken my questions into several threads.In this message I'm going to ask you two questions(Thanks in advance even if you don't answer)but befor that please make all your explanations related to BE only...
1- I`ve read these two sentenses
"I should invite you if I had enough money"
"I would invite you if I had enough money",
what's the difference between the two sentences?
There can be the difference pointed out by Davy, an deontic 'should' wherein, opinions of advise/mild obligation are described, but there is also an epistemic 'should' [level of certainty] that is almost an equal to an epistemic [level of certainty] 'would'.
If you came, I should/would enjoy that very much.
If she makes apple pie, I should/would be delighted.
This use of 'should' is not very common, especially in NaE. It is more common in BrE but I can't say how common. It sounds rather formal.
& when must we use only should in conditional sentences type2?&
when must we use only would in conditional sentences type2/3?& when can we use either should or would in conditional sentences type2/3?
Besides those dialectal differences I noted above, there is no time when we 'must' use any modal except to state/describe the meaning/modal meaning we intend, Bayan.