Learn Spanish (Spain): Ella es la tercera hija de la familia. - She is the third daughter of the family.

The rule about dropping the final letter (making it tercer) only applies before masculine singular nouns, like el tercer hijo. Because hija is feminine, we use the regular feminine form tercera.

Why does tercera come before hija? Don't Spanish adjectives usually go after the noun?

Yes, most descriptive adjectives go after the noun. However, ordinal numbers (like first, second, and third) are an exception. They almost always come before the noun they describe, just like they do in English.

Could I just use the normal number and say la tres hija?

No, Spanish makes a strict difference between counting numbers (uno, dos, tres) and order numbers (primero, segundo, tercero). To say third, you must use the ordinal number tercera.

Since primero and tercero drop their final letter for masculine nouns, does segundo do that too?

No, it does not! Only primero (first) and tercero (third) have this special rule where they drop the -o before a masculine singular noun. Segundo (second) never drops its ending, so you would say el segundo hijo.