ليس

1. Basic Function
In a standard nominal sentence, you have a subject and a predicate (e.g., The house is big). To negate this, you place ليس at the beginning: ليس البيتُ كبيراً (The house is not big). In Arabic grammar, the subject is called اسم ليس (Ism Laysa) and the predicate is called خبر ليس (Khabar Laysa).

2. The Case Change
One of the most important rules of ليس is that it affects the predicate (the description). The predicate must change from the nominative (-u) to the accusative case (-a).

• Positive: الطقسُ باردٌ (The weather is cold)
• Negative: ليسَ الطقسُ بارداً (The weather is not cold)

3. A Static Verb
ليس is a 'static' verb. This means it only exists in the past-tense form, although it normally negates a present state; context can place that state at another time. The middle letter ي is dropped whenever the suffix causes the final letter 'sīn' to carry a sukūn. This happens in the 1st and 2nd person, and the 3rd person feminine plural:

• I am not: لَسْتُ (lastu)
• You (masc.) are not: لَسْتَ (lasta)
• They (fem. plural) are not: لَسْنَ (lasna)
• She is not: لَيْسَتْ (laysat)

4. Emphatic Negation with بـ
A very common feature in Modern Standard Arabic is adding the preposition بـ to the predicate for emphasis. When this happens, the predicate takes the genitive case (-i) instead of the accusative (-a).
Example: ليس الامتحانُ بِصَعْبٍ (The exam is not difficult [at all]).

5. Negating Existence and Possession
To say "there is not," you can combine ليس with هناك or a prepositional phrase like في (in) or عند (at/having). In these sentences, the subject usually becomes indefinite and moves to the end of the sentence, but it still keeps the nominative (-u) case.

• ليس هناك وقتٌ (There is no time)
• ليس في البيتِ أحدٌ (There is no one in the house)
• ليس عندي مالٌ (I do not have [any] money)

While the masculine ليس is common for general existence, it often agrees with the subject if that subject is feminine (e.g., ليست هناك مشكلةٌ - There is no problem).

Position and Visible Case: In ليس الطقسُ باردًا, الطقسُ is the nominative subject and باردًا is the accusative predicate. With ليس الطقسُ بباردٍ, the added ب makes باردٍ visibly genitive; the predicate still occupies the accusative syntactic position (منصوب محلًّا). A phrase such as في المدرسة has no single adjective ending to change.