"A future result or consequence is expressed by the subjunctive in such instances as these: 'I will wait till he return'; 'no fear lest dinner cool'; 'thou shalt stone him with stones, that he die'; 'take heed lest at any time your hearts be overcharged with surfeiting.'
"Uncertainty as to a past event may arise from our own ignorance, in which case the subjunctive is properly employed, and serves the useful purpose of distinguishing our ignorance from our knowledge. 'If any of my readers has looked with so little attention upon the world around him'; this would mean—'as I know that they have.' The meaning intended is probably—'as I do not know whether they have or not,' and therefore the subjunctive 'have' is preferable. 'If ignorance is bliss,' which I (ironically) admit. Had Gray been speaking seriously, he would have said, 'if ignorance be bliss,' he himself dissenting from the proposition.
"A wish contrary to the fact takes the subjunctive: 'I wish he were here' (which he is not).
"An intention not yet carried out is also subjunctive: 'The sentence is that you be imprisoned.'
"The only correct form of the future subjunctive is—'if I should.' We may say, 'I do not know whether or not I shall come'; but 'if I shall come,' expressing a condition, is not an English construction. 'If he will' has a real meaning, as being the present subjunctive of the verb 'will': 'if he be willing,' 'if he have the will.' It is in accordance with good usage to express a future subjunctive meaning by a present tense; but in that case the form must be strictly subjunctive, and not indicative. 'If any member absents himself, he shall forfeit a penny for the use of the club'; this ought to be either 'absent,' or 'should absent.' 'If thou neglectest or doest unwillingly what I command thee, I will rack thee with old cramps'; better, 'if thou neglect or do unwillingly,' or 'if thou should neglect.' The indicative would be justified by the speaker's belief that the supposition is sure to turn out to be the fact.
"The past subjunctive may imply denial; as, 'if the book were in the library (as it is not), it should be at your service.'
"'If the book be in the library,' means, 'I do not know whether it be or not.' We have thus the power of discriminating three different suppositions. 'If the book is in the library' (as I know it is); 'if it be' (I am uncertain); 'if it were' (as I know it is not). So, 'if it rains,' 'if it rain,' 'if it rained.' 'Nay, and the villains march wide between the legs, as if they had gyves on,' implying that they had not.
"The same power of the past tense is exemplified in 'if I could, I would,' which means, 'I can not'; whereas, 'if I can, I will,' means 'I do not know.'
"The past subjunctive may be expressed by an inversion: 'Had I the power,' 'were I as I have been.'
"In Principal Clauses.—The principal clause in a conditional statement also takes the subjunctive form when it refers to what is future and contingent, and when it refers to what is past and uncertain, or denied. 'If he should try, he would succeed'; 'if I had seen him, I should have asked him.'