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Questions & Answers about این درسِ هفتم است.
Why is there an Ezafe sound (-e) between dars (lesson) and haftom (seventh)?
Ordinal numbers ending in -om act exactly like adjectives. Since adjectives come after the noun in Farsi, they must be linked to the noun with an Ezafe. Therefore, "the seventh lesson" is structured as "lesson-[e] seventh" (dars-e haftom).
How is the word haftom (seventh) created?
You simply take the normal number haft (seven) and add the suffix -om. This works for almost all numbers to make them ordinal (like second, third, fourth), with the main exception being "first" (avval).
Would the meaning change if I didn't pronounce the Ezafe and said in dars haftom ast?
Yes, it changes the grammar completely! With the Ezafe (in dars-e haftom ast), the word in (this) is the main subject: "This is the seventh lesson." Without the Ezafe (in dars haftom ast), the phrase in dars (this lesson) becomes the subject, meaning: "This lesson is the seventh."
How would a native speaker say this in a casual, everyday conversation?
In spoken Farsi, the formal verb ast (is) is almost always shortened to an -e sound attached to the end of the previous word. A native speaker would say in dars-e haftom-e.