The word "كأس" (kās) is a feminine noun in Arabic. It can be used as a subject, object, or predicate.
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Subject: When used as the subject of a sentence, it is in the nominative case. For example, "أحضرت كأساً من الماء" (ʾaḥḍatht kāsan min al-māʾ) - "I brought a glass of water."
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Object: When used as the object of a sentence, it is in the accusative case. For example, "شربت كأساً من الماء" (sharabtu kāsan min al-māʾ) - "I drank a glass of water."
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Predicate: When used as the predicate of a sentence, it is in the nominative case. For example, "هذا كأس" (hāḏā kās) - "This is a glass."
In some cases, the word "كأس" can also be used as a vocative. For example, "يا كأس" (yā kās) - "Oh, glass!"
Here are some examples of how the word "كأس" can be used in a sentence:
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"أحضرت كأساً من الماء لضيوفي." (ʾaḥḍatht kāsan min al-māʾ liḍiyāfī) - "I brought a glass of water for my guests."
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"هل يوجد كأسان في الثلاجة؟" (hal yajidūn kāsān fī al-talqāfa) - "Are there two glasses in the refrigerator?"
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"اشرب كأساً من العصير." (ʾiṣrab kāsan min al-ʿuṣīr) - "Drink a glass of juice."
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"هذا كأس جميل جداً." (hāḏā kās jamīl jiddan) - "This is a very beautiful glass."
I hope this helps!