a. We practiced the song every day. And we could the song well
b. He left his country two days later. One year later, he could land the beautiful country.
c. After he came back home, he built up himself. He could swim long time.
d. I’m glad I could finally come to Japan.

As for “could”, are these usage of “could” accurate? Or should I change them into other expressions such as “be able to”, adverbs, verbs, or others.
I want to know natural expressions.

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a. We practiced the song every day and we could sing the song well.
b. He left his country two days later. It would be a year later until he would return to that beautiful country.
c. Once back home he built himself up. He became strong and could now swim for long periods of time.

a. and now we can sing the song very well. we are able to sing the song well.
b. It would be a year later when he would be able to return to this beautiful country.
c. He is able to swim a long time. He can now swim for long periods of time.
d. I'm glad I was finally able to come to Japan. I'm glad I had the opportunity to finally come to Japan. I'm glad I finally had the chance to come to Japan. I'm glad I finally had the ability to come to Japan.