ظرف الزمان 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.

أدرس ليلاً.

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.

هذا صباحٌ جميل.

This is a lovely morning.

Some of the words used as (ظرف الزمان) cannot take any case mark or vowelling, e.g.

منذُ- مذْ  = since

أمسِ  = yesterday

الآنَ = now

قطُّ = never

لمّا = when

ريثَ = while

كلّما = whenever