A common phrase in Arabic which is found in the call to prayer is حَيَّ على الصلاةwhich means more or less “come to prayer!”
In english this word sounds more like a command (eat, please sit, etc.) than a normal noun (book, chair, etc.)
However, a student of Arabic who has not been exposed to the word حيَّ would find it to be an abnormal command. Usually commands end with a sukoon (دَرِّسْ), have a hamzatul wasl before it (اِجْلِسْ), or in some situations it will end with vowel due to a weak letter (ا،و،ي) dropping such as اِمْشِ where the ي dropped as indicated by the kasra.
حَيَّ has none of these characteristics. So how do we understand it?
حَيَّ is a special word called an اسم فعل أمر, which is an noun that has the meaning of a command. (The full i’raab can be seen below)
اسم فعل أمر بمعنى أقبلْ، مبني على الفتح لا محل له من الإعراب، وفاعله ضمير مستتر فيه وجوبا تقديره أنت
It is a command with the meaning of to approach or to come. So now let’s look at the phrase:
حَيَّ على الصلاة
Come to prayer