2. In concessions and conditions ([§§ 279–280]).
- Though {I | you | he} should fail, there would still be hope.
- If {I | you | he} should fail, all would be lost.
3. In sentences expressing not what is or was, but what would be or would have been, the case ([§ 283]).
- {I should | You would | He would} write to Charles if I knew his address.
- It would have been better to telegraph.
4. In subordinate clauses introduced by that, lest, before, until, etc. ([§ 284]).
- I will take care that nothing may prevent.
- I took care that nothing {might | should} prevent.
- The general determined to wait until fresh troops should arrive.
POTENTIAL VERB-PHRASES
USE OF MODAL AUXILIARIES
287. Several auxiliary verbs are used to form verb-phrases indicating ability, possibility, obligation, or necessity.
Such verb-phrases are called potential phrases, that is, “phrases of possibility.”