Abarim Publications' online Biblical Hebrew Dictionary
לוא לא לו
The form לוא (lw') is either an expanded version of the particle of negation לא (lo') and means no, or it is an expanded version of the particle of entreaty לו (lu) and means if:
The particle לא (lo') or לוא (lo') is the Hebrew primary particle of prohibition. There are some others (a set perhaps comparable to the English "not", "un-", "dis-" etc). It's the Hebrew non-negotiable "no-that-means-no", which obviously occurs all over the Bible, from Genesis 2:5 to the Ten Commandments in Exodus 20 and Deuteronomy 5, and up to Malachi 4:1.
On rare occasions our particle becomes a noun meaning nothing or nought (Job 6:21), and often it occurs combined with other particles:
- בלא (belo) literally means "in not", and may be translated with without (Jeremiah 22:13), not through (Job 30:28), outside of (Leviticus 15:25), before (Job 15:32).
- הלא (helo) appears to reflect a rhetorical question, inviting an affirmative answer: is/are not? (Genesis 13:19, Deuteronomy 3:11, 1 Kings 11:41).
- ולא (wlo) combines the particle of conjunction with our particle of negation: and not, or: and if not, etc (2 Samuel 13:26, Obadiah 1:16).
- ללא (lelo) literally means (in the elegant words of BDB Theological Dictionary): "in the condition of no" (2 Chronicles 13:9, Job 26:2, Isaiah 65:1).
The particle of entreaty לו (lu) or לוא (lu', spelled the same as the secondary form of the previous word) either states a comparative hypothetical case (if, or if it were that; Deuteronomy 32:29, Ezekiel 14:15, Job 16:4) or expresses a desired hypothetical case (if it only were that; Genesis 17:18, Numbers 14:2, Isaiah 48:18).
Combined with the particle of negation לא (lo') described above, it forms the particle of negative entreaty לולא (lule'), also spelled לולי, which expresses a case in which a non existing situation is compared to an existing one (if not, unless, if it weren't that; Deuteronomy 32:27, Judges 14:18, Psalm 124:1).