occur and run

Occur, concur, incur all come from the IE root of run, as in a horse runs. And like correr, in Spanish, they all have been Latinized.

The English run is from the similar IE root to flow. It never got Latinized.  So we still have run, ran; the IE form of conjugation. Weep, wept, sing,sang, sung; ring, rang, rung. This is how the pre-literate done their tense changes. They likely had children's songs to make their grammar stable.

Eat is another that skipped Latin and sounds like it was mangled in Old English.

Past tense, -ed

That comes from the past participle, I am pooped, from the Latin and is like Spanish -ado.  But is got converted to a verb, in the past tense, somehow. So id slept and wept which were sleeped and weeped, like blessed; fully pronounced. This sounds like a cheat, borrowing part of the Latin system to get a cheap past tense as a suffix.