Spanish has many different irregulars in its preterite tense (preterite is what we call it in Spanish class, I think the linguist term is simple past but I might be wrong), most of which involve a change in the stem of the verb, for example:
Hacer (to do): Haces -> Hiciste
Andar (to walk): Andas -> Anduviste
Querer (to want): Quieres* -> Quisiste
Poner (to put): Pones -> Pusiste
All of these forms are in the second person singular, indicative mood, going from Present -> Preterite.
*Querer is what is known as a “stem changer” in the present, if it wasn’t a stem changer it’s second person present form would be *queres.
These irregulars take what are known as “irregular endings” which are:
Sing. Pl. 1p -e -imos 2p -iste -isteis 3p -o -ieron
This is opposed to "regular endings" which are:
(-ar infinitive) (-er/-ir infinitive) Sing. Pl. Sing. Pl. 1p -é -amos -í -imos 2p -aste -asteis -iste -isteis 3p -ó -aron -ió -ieron
Note that in the regular conjugation the 1st and 3rd person singular have syllable final stress, which in the -ar infinitive conjugation class is the only thing distinguishing it from the 1st person singular present (e.g., hablo vs habló).
It reminded me of Germanic strong verbs as the stem vowel is changing in each situation, but I know it’s a far chance for those two systems of inflection to be related. So what is the reason for this irregularity in the past? If this is expected from Latin (as comments suggest) then why did this discrepancy exist in Latin? Is it actually traceable back to PIE ablaut?
Note: I am not asking about the verbs ser, ir, dar, or ver. Those are also irregulars (as in they do not have syllable final stress in the 1ps and 3ps) but do not follow this same pattern as they don’t take the irregular endings.