is a special category of nouns in Arabic used in a sentence to denote time. It is a special type of noun in Arabic, but it can be understood as an adverbial of time, as it gives the time a certain action takes place. It is the word that answers the question: “when did the action take place”? Consider the following examples:
أتناول الفطور صباحاً.
I have breakfast in the morning.
I study at night.
أعمل في المكتبة يوم الاثنين.
I work at the library on Monday.
ظرف الزمان is usually accusative (منصوب).
Some examples of ظرف الزمان include (حين = when, صباح = morning, ظُهر = noon, سنة = year, etc.)
It should be noted that if any of these words is used in the general sense not as the time of a certain action, then it should be case marked according to its function in the sentence, e.g.