Learn Farsi: امروز جلسه‌ی چهارم کلاس ریاضی است. - Today is the fourth session of the math class.

It is formed by adding the suffix -om to the base number چهار (chahār - four). In Farsi, Type 1 ordinal numbers (those ending in -om) act exactly like regular adjectives. This means they must come after the noun they describe and are linked to it using an Ezafe, as seen in جلسه‌ی چهارم (the fourth session).

Why does the word جلسه (session) have a ‌ی added to the end of it here?

The word جلسه (jalese) ends in a silent "heh" letter, which makes an "e" sound. When we need to add an Ezafe to connect it to the next word (چهارم), two vowels would clash. To fix this, the Ezafe is pronounced as "-ye" instead of "-e", and we write it by adding a ی to make جلسه‌ی (jalese-ye).

How does the long Ezafe chain work in جلسه‌ی چهارم کلاس ریاضی?

The Ezafe particle links words together to show either description (like an adjective) or possession (like "of"). Here, it does both in a chain: جلسه‌ی (session) connects to its adjective چهارم (fourth), which connects to کلاس (class), which connects to its modifier ریاضی (math). The whole chain translates smoothly to "the fourth session of the math class".

How would the final part ریاضی است (riāzi ast) sound in casual, spoken Farsi?

In everyday speech, the "is" verb است (ast) is almost always shortened to an "-e" sound attached to the previous word. Because (riāzi) already ends in a vowel sound ("i"), spoken Farsi adds a "y" bridge to connect them. So, instead of "riāzi ast", you will hear people say (riāzi-ye).