It may have occurred to the reader that our ordinal series is rather odd.
Look; fourth, fifth, sixth, etc. make some sort of sense because they’re just the cardinal number followed by -th.
However, first, second, and to a lesser degree third, are odd.
Here’s the story.
Think of the word ‘former’; it means “what came before”, or something to that effect. It comes from Old English “Forman”, e.g. ‘forman sithe’ means “the first since”, or “the most recent”. So, by a bit of bad pronunciation, we can see that the fir- part of fir-st comes from ‘former’. Now, consider degrees of comparison; we have -st to indicate extremes. So; largest = most large; most = most more (more-est), fattest = most fat, etc. So, most former, formerest = first. Just a bit of bad pronunciation or laziness led to ‘first’ from ‘former’st’. Interestingly, Swedish has ‘först’ as well, pronounced the same.
Now, consider the Afrikaans word, ‘eerst’. It seems strange, but it’s not. In early modern English we have the word ‘ere’, meaning ‘before’. It survives in ‘early’ - meaning ‘before-like’. Similarly, ‘erstwhile’ - meaning ‘at first’ or ‘previously’. Erst being, then a combination of the extreme -st ending and 'ere.’ I seem to recall that the Old English was Aer. At any rate, why then did English favour ‘former’st’ rather than ‘erst’? I can’t tell. Perhaps ‘erst’ seemed too ambiguous?
Then let’s look at second. This one’s easy; it’s straight from Latin secundus. However, why would English have chosen that over, say, twoth, or twost? In German we have zweite, Afrikaans we have tweede, so we should have twoth or twooth or tweeth. But I suppose with the way we pronounce ‘two’ (too), it would sound like ‘tooth’, or ’teeth' and therefore be ambiguous. So it was discarded.
Then third. Again, this is easy. Vowels and R often swap around; consider OE “brid” (bird), or OE Thurh (Thru). So, ’thri’ (3) in old english, when made into an ordinal, became ’thrid’ or ’thri∂', which is easy to see how it would become ’third’.