"Tidy up the school, Gareth."
Translation:Taclusa'r ysgol, Gareth.
As the sentence is addressed to a particular individual (Gareth), would the plural command form be used here?
Since we are talking about one individual you can use either the form "Taclusa" or "Tacluswch" here, though remember "Taclusa" is informal and "Tacluswch" is formal. If we were talking to a group you would have to use "Tacluswch".