Please help me explain to a non-English speaker why "I think it should be tore down" is incorrect and she should say instead "I think it should be torn down." She thinks since the object is still standing, the past tense of "tear" (torn) should not be used. I can't find a way to explain why this seeming logic is incorrect.
Torn is not the past tense of tear: it is the past participle. (The past tense is tore.)
And that name, ‘past’ participle, is not descriptive, only a historical accident: English participles in fact have no tense. In most contexts it might better be named a passive participle, because the English passive is constructed with BE+‘past’ participle; or a perfect participle, because the English perfect is constructed with HAVE+‘past’ participle.
The ‘past’ participle is obligatory in those constructions, but it does not indicate any particular time reference; only the first auxiliary in a verb group bears tense—
- It is torn down ... ‘present’ passive
- It was torn down ... ‘past’ passive
- He has torn it down ... ‘present’ perfect
- He had torn it down ... ‘past’ perfect
And of course even in these constructions the terms ‘past’ and ‘present’ (and, for that matter, ‘tense’) are not always appropriate: the ‘present’ may have future reference, and the ‘past’ may have present reference with the shift indicating some degree of unreality. This is most evident with modal verbs, whose ‘past’ forms have nonpast irrealis reference more often than not:
- It should(past) be torn down means tearing it down has not yet happened but is a desirable future project.