Latin-derived verbs

cost

Comes from the Latin "constare", meaning literally to stand with, whose meaning likely changed due to the expression "The item stands at X dollars", according to etymonline.

There are surely others.

I like etymonline.com for these questions.

For example, it tracks 'spend' from Old English from Latin. This sounds right to me, because spend-spent sounds like a Germanic verb conjugation. There's also rend-rent, lend-lent and bend-bent. You can make groups of these and you'll find that they all have similar histories.

Bend from bendan spend from -spendan (from forspendan, to use up) rend from rendan lend - laenan

So it seems like those old English "-an" verbs that we still have get conjugated into the past with "-t".

But tend-tended, eh? Etymonline again to the rescue: Either middle english "Attenden" or old French "tendre". Not old English Tendan!

So the best answer to your question I can give is, if you want to figure out why a verb is conjugated the way it is, think of as many verbs with similar endings, make a list comparing their present forms and the other forms you're interested in, and then look up the etymologies! Any "rules" will become more and more clear as you go on.

Hope that helps!