The last tense, and the messiest. This is the one English usually marks with -ed: wanted, walked, or the irregulars like ran, ate. It is more random than the other tenses, though there is still some logic to find.
hablé (I spoke) · habló (he/she spoke)
Take off the -ar, add the ending, and stress it. Use hablar as your hook verb to look back to.
Compré · Lo compré · No lo compré · Lo compró · Me lo compró · Envié · Lo envié · Te lo envié · No te lo envié · Olvidé · Olvidó
This is the trap of the tense. You have spent the whole course associating -o with I, but here I is -é and -ó is he, she, it and formal you.
This is exactly why the accent mattered so much in the present tense. Stress it wrongly and you have said something else entirely.
No hablé (I didn't speak) against no habló (he didn't speak)
When a "to" form does not feel right, cycle through the three endings. The one you have heard before will jump out at you.
"comprir", "comprer", comprar · the last one is the one you know
English says it was sent yesterday without saying who did it. Spanish says it sent itself.
Se envió ayer. (it was sent yesterday)
El olvido has no real English equivalent: the process, the experience of forgetting. You will hear it a lot in songs, especially in tango.
No te lo envié todavía · Lo envió ayer · Habló conmigo · No sé por qué no habló conmigo · Lo iba a hacer pero lo olvidé
Todavía is toda (all) plus vía (way), so "all the way". No Spanish speaker hears that, any more than we hear as and well inside as well.
Curious about a word's origin? etymonline.com is a good free etymological dictionary.