"I swam to school in 1970."
Translation:Ro'n i'n nofio i'r ysgol ym mil naw saith dim.
I'm afraid that this is a zombie sentence so I can't see what were the accepted answers. Though whilst it is possible to drop the pronoun in the sentence you suggested it's rather uncommon in the modern language so it would be "Nofiais i i'r ysgol ym mil naw saith dim."
Yes, at least on this course. The dropping of pronouns is normal in the formal registers of the language, but they are almost invariably kept in in the modern colloquial language, as EllisV says.
Because this course is aimed at learners, we do not use, or accept as answers, patterns in the formal register.
We should have pointed out the likely reason why the sentence was deleted...
I swam... would point towards the simple past tense rather than the imperfect as was put in the answer, as does putting a particular date in the Welsh sentence. The Welsh should have been:
- [Nofiais i/Gwnes i nofio] i'r ysgol [unwaith/sawl gwaith/...] ym mil naw saith dim, or,
- Ro'n i'n nofio i'r ysgol [pan o'n i'n ifanc] - 'I used to swim to school [when I was young]' (implying something habitual in the past).