It is one of many idioms that come from la vez (pl veces). Other examples: de vez en cuando (once in a while or from time to time); dos veces (twice); en vez de (instead of); muchas vez (often or many times); otra vez (again); raras veces (rarely, seldom); tal vez (maybe); una vez (once).
"A veces" does not mean "sometimes I." It means "sometimes" (the literal translation is "at times"). "Corro" means "I run." Unlike English, which has pretty poor morphology, the verbs in Spanish include who is doing the action.
I've bolded the parts that correspond to each other:
Corro -- I run
Corres -- you run (singular, informal "you")
Corre -- he/she/it runs; you run (singular, formal "you")
Corremos -- we run
Corréis -- you run (plural, informal "you" --- this is used pretty much only in Spain)
Corren -- they/you run (plural, formal "you")
It does make me frustrated when they don't explain things and then expect us to know. There are so many times where I'll add something because it seems to make sense when translated to English and they will count it wrong and say I added a word. Then, in this case, I didn't add "on" because I didn't see a word that meant "on". I thought it didn't really make sense, but it says "Sometimes I run Tuesdays." and, of course, this time we are supposed to understand that "on" is to be added. It's very frustrating.
Hola rolandoligno, well, If you put
-"Occasionally=A veces, de vez en cuando"
-"Sometime=Algunas veces, en algún momento.
If we look at the difference In Its meaning is a little small, but the context says on Tuesday, so, I can realize that It is not "Occasionally" but "Sometimes" I have a frequency doing exercise.
I hope to have you helped f there are doubts or mistakes please comment
Greetings and luck