Though the question has been covered before, perhaps the regional and registral variations could be better addressed. Paraphrases are shown using equals signs:
- [A1] She insisted that we attend the party. [subjunctive mandative, = [A3] She insisted that we should attend the party, or possibly indicative with present simple for ongoing iterative situation: She insisted that we regularly attend the party. (the party a repeating event; use of distributive singular)]
- [A2] She insisted that we attended the party. [covert mandative with past simple, = She insisted that we should attend the party, or indicative, = She insisted that we had attended the party.]
The above, see [CGEL {Huddleston & Pullum, 2002, 7.1.1], which adds the caveat
Clear cases of the covert construction are fairly rare, and indeed in
'AmE' [my scare quotes] are of somewhat marginal acceptability. In AmE
the subjunctive is strongly favoured over the should construction,
while 'BrE' shows the opposite preference.
So 'should we say' has no unique answer; the mandative sense would be seen as needing the subjunctive (A1) by the vast majority of Americans, while most Brits would prefer the simple past covert (A2) or the periphrastic should (A3) alternatives (with, I'd say, A2 the more usual, especially in conversation).
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For the sake of completeness, the other ways of expressing the mandation listed:
- [B] She insisted that we should attend the party. [uses declarative content clause = that-clause] [common]
- [C1] She insisted on us attending the party. [uses ing-clause; ACC-ing construction]
- [C2] She insisted on our attending the party. [uses ing-clause; POSS-ing construction] [sounds rarefied in conversation, in the UK at least]