I wonder why they are so late. They should have arrived a long time ago.
Does this sentence have two possible meanings? How would we choose between them?
1) I expected (predicted) their arrival would have taken place a long time ago.
2) It would have been better for them to arrive much earlier, maybe they have missed something at the party.
Answer
The ambiguity is real, but in practice will be overcome by context.
In the context of I wonder why they are so late the natural interpretation of should is epistemic referring to probability or expectation: your 1).
I wonder why they are so late. They should have arrived long ago. I hope nothing’s happened …
But it in another context the natural interpretation might be deontic, referring to obligation or advisability: your 2).
What a pity they’re arriving now. They should have arrived a long time ago, when we were having fun. Now the party’s started breaking up.
And then of course it’s possible that both meanings are intended.
What a pity they’re so late. They should have arrived a long time ago, when the party was rocking. Gosh, you don’t suppose something’s happened to them?
Attribution
Source : Link , Question Author : Graduate , Answer Author : StoneyB on hiatus
