First, Second, etc.
Now that you're familiar with Sanskrit numbers, we can study words like "first" and "second." These words are derived from the numbers we just saw, and they are used as adjectives.
First to Tenth
As mentioned above, these numbers are used as adjectives. These words are often used to number the chapters of a work:
अथ प्रथमो ऽध्यायः । अर्जुनविषादयोगः
atha prathamo 'dhyāyaḥ । arjuna-viṣāda-yogaḥ
Now starts the first chapter : the yoga of Arjuna's despair
In English, the words from "fourth" onward mostly use "-th", but the words from "first" to "third" do not. In the same way, the words from "fourth" onward use ī for the feminine, but the words from "first" to "third" do not. This correspondence is a coincidence, but it's a useful way to remember which consonants use ī in the feminine.
11th to 19th
The number remains the same. It is inflected like a noun ending in -a. The feminine uses ī.
अथ षोडशो ऽध्यायः । दैवासुरसम्पद्विभागयोगः
atha ṣoḍaśo 'dhyāyaḥ । daiva-asura-sampad-vibhāga-yogaḥ
Now starts the sixteenth chapter : the yoga of the division between divine and demonic fates
20th to 99th
The number either has tama added to the end of it or is "contracted" to a shorter form.
एकविंश, चतुस्त्रिंश, नवचत्वारिंश, अष्टापञ्चाश, द्वाषष्ट, सप्तसप्तत, पञ्चाशीत, षड्णवत
ekaviṃśa, catustriṃśa, navacatvāriṃśa, aṣṭāpañcāśa, dvāṣaṣṭa, saptasaptata, pañcāśīta, ṣaḍṇavata
twenty-first, thirty-fourth, forty-ninth, fifty-eighth, sixty-second, seventy-seventh, eighty-fifth, ninety-sixth
The number either has tama added to the end of it or remains the same.