The uses of the Subjunctive in principal clauses are as follows:

160. The Hortatory Subjunctive. The Subjunctive is used in the first person plural in exhortations, the speaker thus exhorting others to join him in the doing of an action. HA. 866, 1; G. 1344; B. p. 209; WM. p. 355; G.MT. 255, 256.

Heb. 12:1; diV u`pomonh/j tre,cwmen to.n prokei,menon h`mi/n avgw/na, let us run with patience the race that is set before us.

1 John 4:7; avgaphtoi,( avgapw/men avllh,louj, beloved, let us love one another.

161. Occasionally the first person singular is used with a;fej or deu/ro prefixed, the exhortation in that case becoming a request of the speaker to the person addressed to permit him to do something.
Matt. 7:4; a;fej evkba,lw to. ka,rfoj evk tou/ ovfqalmou/ sou, let me cast out the mote out of thine eye. See also Luke 6:42; Acts 7:34.
The sense of  a;fej in Matt. 27:49 and of  a;fete in Mark 15: 36 is doubtful (see R.V. ad loc. and Th., avfi,hmi, 2, E.).

In Matt. 21:38 (Mark 12:7) deu/teis prefixed to a hortatory first person plural without affecting the meaning of the Subjunctive.

In none of these cases is a conjunction to be supplied before the Subjunctive. Cf. the use of a;ge( fe,re, etc., in classical Greek. G.MT. 257; B. p. 210; WM. p. 356.

162. The Prohibitory Subjunctive. The Aorist Subjunctive is used in the second person with mh, to express a prohibition or a negative entreaty. HA. 866, 2; G. 1346; G.MT. 259.

Matt. 6:34; mh. ou=n merimnh,shte eivj th.n au;rion, be not therefore anxious for the morrow.

Heb. 3:8; mh. sklhru,nhte ta.j kardi,aj u`mw/n, harden not your hearts.

Matt. 6:13; kai. mh. eivsene,gkh|j h`ma/j eivj peirasmo,n, and bring us not into temptation.

163. Prohibitions are expressed either by the Aorist Subjunctive or by the Present Imperative, the only exceptions being a few instances of the third person Aorist Imperative with mh,. The difference between an Aorist Subjunctive with  mh, and a Present Imperative with mh, is in the conception of the action as respects its progress. HA. 874. Thus

164. (a) The Aorist Subjunctive forbids the action as a simple event with reference to the action as a whole or to its inception, and is most frequently used when the action has not been begun.

Acts 18:9; la,lei kai. mh. siwph,sh|j , speak and hold not thy peace.

Rev. 7:3; Mh. avdikh,shte th.n gh/n, hurt not the earth.

165. (b) The Present Imperative (180-184) forbids the continuance of the action, most frequently when it is already in progress; in this case, it is a demand to desist from the action.
Mark 6:50; evgw, eivmi\ mh. fobei/sqe, it is I, be not afraid.

John 5:14; mhke,ti a`ma,rtane, sin no more.

When the action is not yet begun, it enjoins continued abstinence from it.
Mark 13:21; kai. to,te eva,n tij u`mi/n ei;ph|( :Ide w-de o` Cristo,j( :Ide evkei/( mh. pisteu,ete, and then if any man shall say unto you, Lo, here is the Christ; or, Lo, there; believe it not. Cf. Matt. 24:23.
166. The Prohibitory Subjunctive occurs rarely in the third person. 1 Cor. 16:11; 2 Thess. 2:3.

167. The strong negative, ouv mh, occurs rarely in prohibitions with the Aorist Subjunctive.

Matt. 13:14 and Acts 28:26, from Septuagint, Isa. 6:9, are probably to be understood as prohibitory (as in the Hebrew of the passage in Isa.), rather than emphatically predictive, as in RV. Cf. Gen. 3:1, ouv mh. fa,ghte, which is clearly prohibitory. G.MT. 297. Cf. 162.

In Matt. 21:19, on the other hand, the emphatic predictive sense, there shall be no fruit from thee hence forward forever, is more probable, being more consistent with general usage and entirely appropriate to the context. The imperative rendering of the R .V. makes the passage doubly exceptional, the Imperative Subjunctive being rare in the third person, and ouv mh, being unusual in prohibitions.

168. The Deliberative Subjunctive. The Subjunctive is used in deliberative questions and in rhetorical questions having reference to the future. HA. 866, 3; G. 1358.

Luke 3:10; Ti, ou=n poih,swmen, what then shall we do?

Luke 11:5; Ti,j evx u`mw/n e[xei fi,lon . . . kai. ei;ph| auvtw|/, which of you shall have a friend . . . and shall say to him?

169. Questions may be classified as questions of fact and questions of deliberation. In the question of fact the speaker asks what is (or was or will be). In the question of deliberation, the speaker asks what he is to do, or what is to be done; it concerns not fact but possibility, desirability, or necessity.  But questions may be classified also as interrogative or real questions, and rhetorical questions. The former makes a real  inquiry (for information or advice) ; the latter is a rhetorical substitute for an assertion, often equivalent to a negative answer to itself, or, if the question is negative, to a positive answer.

Since both questions of fact and questions of deliberation may be either interrogative or rhetorical, it results that there are four classes of questions that require to be distinguished for purposes of interpretation.

(a)    The interrogative question of fact.

Matt. 16:13; Ti,na le,gousin oi` a;nqrwpoi ei=nai to.n ui`o.n tou/ avnqrw,pou, who do men say that the Son of man is? See also Mark 16 : 3; John7:45; Acts 17:18.

(b)    The rhetorical question of fact.

1 Cor. 9:1; ouvk eivmi. avpo,stoloj, am I not an apostle?

Luke 23:31; o[ti eiv evn tw/| u`grw/| xu,lw| tau/ta poiou/sin( evn tw/| xhrw/| ti, ge,nhtai, for if they do these things in a green tree, what will be done in the dry? See also Luke 11:5; 16:11.

(c) The interrogative deliberative question.

Mark 12:14; dw/men h' mh. dw/men, shall we give, or shall we not give? See also Matt.6:31; 18:21; Luke 22:49.

(d) The rhetorical deliberative question.

Rom. 10:14; pw/j ou=n evpikale,swntai eivj o]n ouvk evpi,steusanÈ pw/j de. pisteu,swsin ou- ouvk h;kousanÈ . . . pw/j de. khru,xwsin eva.n mh. avpostalw/sin, how then shall they call on him in whom they have not believed?  how shall they believe in him whom they have not heard? . . . how shall they preach except they be sent? See also Matt. 26:54; Luke 14:34; John 6:68.

Interrogative questions of fact, and rhetorical questions of fact having reference to the present or past, employ the tenses and moods as they are used in simple declarative sentences. Rhetorical questions of fact having reference to the future, and all deliberative questions, use either the Subjunctive or the Future Indicative.

170. The verb of a deliberative question is most frequently in the first person; but occasionally in the second or third.  Matt. 23:33; Rom. 10:14. The verb of a rhetorical question may be of any person.

171. The Deliberative Subjunctive is sometimes preceded by qe,leij( qe,lete( or bou,lesqe. No conjunction is to be supplied in these cases. The verb qe,lein is sometimes followed by a clause introduced by i[na, but i[na never occurs when the verb  qe,lein is in the second person, and the following verb in the first person, i.e. when the relations of the verbs are such as to make a Deliberative Subjunctive probable.

Luke 22:9; Pou/ qe,leij e`toima,swmen,where wilt thou that we make ready? See also Matt. 26:7; 27:17, 21; Mark 10:30, 51; 14:12; 15:9; Luke 9:54; 18:41; 1 Cor. 4:21 (N.B.), and cf. (i[na) Matt. 7:12; Mark 6:25; Luke 6:31; 1 Cor. 14:5.
172. The Subjunctive in Negative Assertions. The Aorist Subjunctive is used with ouv mh, in the sense of an emphatic Future Indicative. HA. 1032; G. 1360.
Heb. 13:5; ouv mh, se avnw/ ouvdV ouv mh, se evgkatali,pw, I will in no wise fail thee, neither will I in any wise forsake thee. See also Matt. 5:18; Mark 13:30; Luke 9:27, et freq. Cf. Gild. in A.J.P. III. 202 f.
REM. In Luke 18:7 and Lev. 15:4 the Subjunctive with ouv mh, is used in a rhetorical question. The Subjunctive may be explained as occasioned by the emphatic negative or by the rhetorical nature of the question.

173. This emphatically predictive Subjunctive is of frequent occurrence in Hellenistic Greek. The Present Subjunctive is sometimes used with ouv mh, in classical Greek, but no instance occurs in the New Testament.  Concerning the rare use of the Future with ouv mh, see 66; cf. Gild. u.s.