Learn Arabic (Modern Standard): جلست على المقعد التاسع.

In Arabic, ordinal numbers like التاسع (ninth) act exactly like adjectives. Because the noun it describes, المقعد (the seat), is specific and has the definite article الـ (the), the ordinal number must also have it to match.

Why does the word for "ninth" come after "the seat" instead of before it?

Since ordinal numbers are treated as adjectives in Arabic, they always follow the noun they describe. The sentence literally says "the seat the ninth" (المقعد التاسع).

How is التاسع (ninth) related to the regular counting number nine?

The regular number for nine is تسعة (tis'a). To make an ordinal number (from 2nd to 10th), Arabic takes the core root letters of the number - in this case t, s, and ' - and stretches them into a standard pattern, making تاسع (taasi').

How do we know جلست means "I sat" instead of "she sat" or "you sat"?

The past tense verb جلس (to sit) changes its ending based on who is acting. The "I" ending is a "tu" sound, while "she" is "at" and "you" (masculine) is "ta". Since short vowel marks usually aren't written, the word looks identical for all three, but general context tells you the speaker means "I sat".