Questions about example sentences with, and the definition and usage of "Ordinal"

A: First, second, third, fourth…

A: 1 (one) - cardinal number
1st (first) - ordinal number

2(two) - 2nd (second)

etc.

A: would you be interested in talking with me to improve my English?

A: We often talk about unknown/undefined numbers as "the n-th time" or "the x-th time".
"N-th" is easier to pronounce with the "th" sound, so it's more common as an ordinal. (The "n" stands for "number".)

...That "n" can then be used as a variable in a math equation.

A: What's your birth order? I'm the 3rd (child) in my family.

A:
Thank You
If the answer is I am the fourth son of my parents, then how to ask the question to get this answer?

A: The one-hundredth (the second « d » is silent)

A: We use THE plus the ordinal numbers when it comes before a noun.

This is THE fourth time I’ve ridden a horse.
THE first man in line was an elderly gentleman.

Fifth grade starts next week

A: Yes.

A: The one thing I'd suggest as a change _in writing_ is to put "pierwsi" and "drudzy" in quotation marks. But that's purely a punctuation thing, so I still rated the English part of it all as perfectly natural. Bookish, but natural. (For example, "exist" would probably become "are" in most people's speech, and "of which" might become "and even as to them.")

A: Muito bom! Parabéns!
Mas acho que "fourth" não soa natural.

Aqui está um audio com um exemplo:
"That was the fourth book I've read this month."

A: 'The' would also be correct here. Using 'a' here puts a tiny bit more focus on 'yuan currency falls' rather than 'second day.' Using 'the' would put more focus on the fact that it is the second day. The nuance is very slight, and it is a choice of style.