"The usual forms of the subjunctive in the principal clause are 'would,' 'should,' 'would have,' 'should have'; and it is to be noted that in this application the second persons take the inflexional ending of the indicative: 'shouldst,' 'wouldst.'
"'If 'twere done when 'tis done, then 'twere (would be) well
It were (should be) done quickly.'
"The English idiom appears sometimes to permit the use of an indicative where we should expect a subjunctive form. 'Many acts, that had been otherwise blamable, were employed'; 'I had fainted, unless I had believed,' etc.
"'Which else lie furled and shrouded in the soul.'
"In 'else' there is implied a conditional clause that would suit 'lie'; or the present may be regarded as a more vivid form of expression. 'Had' may be indicative; just as we sometimes find pluperfect indicative for pluperfect subjunctive in the same circumstances in Latin. We may refer it to the general tendency, as already seen in the uses of 'could,' 'would,' 'should,' etc., to express conditionality by a past tense; or the indicative may be used as a more direct and vivid mode. 'Had' may be subjunctive; 'I had fainted' is, in construction, analogous to 'I should have fainted'; the word for futurity, 'shall,' not being necessary to the sense, is withdrawn, and its past inflexion transferred to 'have.' Compare Germ. würde haben and hätte."
In addition to the foregoing, we find in Professor Bain's "Composition Grammar" the following:
"The case most suited to the subjunctive is contingent futurity, or the expression of an event unknown absolutely, as being still in the future: 'If to-morrow be fine, I will walk with you.'
"'Unless I were prepared,' insinuates pretty strongly that I am or am not prepared, according to the manner of the principal clause.
"'What's a tall man unless he fight?'
"'The sword hath ended him: so shall it thee,
Unless thou yield thee as my prisoner.'