In an English language class at school, I was taught to always use "the" not "a/an" when I want to use an ordinal number, for example, "for the second time in a row" instead of "for a second time in a row."
However, from time to time, I see people use a or an, like "for a second time in a row." And it seems all intentional and correct.
To me, whose native language is not English, both look the same, except only the latter case adds a bit of uncertainty. Is that the case?
Take one more example.
"Life expectancy for Americans declined for the second year in 2021"
- Speaker is 100% sure this is the second time.
"Life expectancy for Americans declined for a second year in 2021"
- Speaker is not 100% sure this is the second time, and he/she does not wish to be blamed later on when it turned out to be the third or fourth time, or;
- Speaker knows it's the second time and is implying that the third or forth time is expected to follow - just a one point in a long series.
This is what I think. How do you switch between "the nth ordinal number" and "a/an ordinal number?"