From my blog: Laysa/ليس, The Nominal Sentence Negator. It is used to negate a nominal sentence that does not contain a verb (see my article on the nominal sentence). ليس Laysa is called “حرف مشبه بالفعل”, a particle resembling a verb, as it is “conjugated” like the verb TO BE, always in the past form. You may already know how to conjugate the verb to be, كان kaana , in the past, but we will review it below. Note how such a verb, containing a weak radical (a long vowel in the root, which makes it irregular) in the middle: · is called معتل الوسط mu3tall alwaSaT, ill in the middle;· keeps the weak radical in the third person conjugation and loses it in the first and second person conjugation. You are probably familiar with verbs of this category: To say: قال، يقول To live: عاش، يعيش To return: عاد، يعود To. sleep: نام، ينام Laysa/ليس also has a long vowel in the middle and will also: lose it in the first and second person conjugation,and keep it in the third person conjugation (see blue highlight)… EXCEPT in the feminine plural, third person (see pink highlight). 

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