أَلَّا
1.
is a particle denoting تَحْضِيض; (Msb in article حض, Mughnee, K;) i. e., when followed by a future, exciting to an action, and seeking or desiring or demanding the performance of it; and when followed by a preterite, reproof for not doing a thing; (Msb ubi suprà;) synonym with هَلَّا; (T, TA;) and peculiar to enunciative verbal propositions, (Mughnee, K,) like the other particles used for the same purpose. (Mughnee.) You say, [أَلَّا تَفْعَلُ كَذَا
Wherefore wilt not thou do such a thing? and] أَلَّا . فَعَلْتَ كَذَا [Wherefore didst not thou such a thing?] (T, TA,) meaning, (TA,) or as though meaning, (T,) لِمَ لَمْ تَفْعَلْ كَذَا. (T, TA.)
2.
It also means أَنْ لَا; the ن being incorporated into the ل, which is written with teshdeed: (T, TA:) in which case, it is not to be confounded with the foregoing particle. (Mughnee.) You say, أَمَرْتُهُ أَلَّا يَفْعَلَ ذَاكَ [I commanded him that he should not do that]; and you may say, أَمَرْتُهُ أَنْ لَا يَفْعَلَ ذَاكَ: it occurs in the old copies of the Qur'an written in the former manner in some places, and in the latter manner in other places. (T, TA.) In the saying in the Qur'an, 27:31, أَلَّا تَعْلُوا عَلَىَّ, [which may mean That ye exalt not yourselves against me, or exalt ye not yourselves against me,] it may be a compound of أَنْ governing a mansoob aorist and the negative لَا, or of the explicative أَنْ and the prohibitive لا. (Mughnee.) [It often has لِ prefixed to it, forming the compound لِيءَلَّا, which signifies That, or in order that,... not; and may frequently be rendered by lest; as in the Qur'an, 2:145, لِيءَلَّا يَكُونَ
لِلنَّاسِ عَلَيْكُمْ حُجَّةٌ
That, or in order that, there may not be, or lest there should be, to men, against you, any allegation.]