"Trevligt att träffas.

Hey Koichi, your short letter is well written. Still, when swedish people present themselves, they prefer to say "i come from" instead of "i'm from", like the english. that means, that "jag kommer från Japan" is more usual and colloquial!

Trevligt att träffas.

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Hej Koichi! Trevligt att träffas! I liked your simplicity in your introduction and how you built your sentences. However maybe "Hallå" could be switched out with "Hej" as "Hej" is more of a greeting than "Hallå" is.

("Hallå" is often used fo attention grabbing or answering the phone. E.x "Hallå du tappade din plånbok!" = "Hey you dropped your wallet!" Or "Hallå? Någon där?" = Hello? Anybody there?" )