"Swimming is a good thing."
Translation:اَلْسِّباحة شَيْء جَيِّد.
سِّباحة without AL was not accepted although there is no THE for the the english word "swimming"
There is a difference between English and Arabic when it comes to translating general nouns or names of general thing.
As an example, take the sentence: I love nature - typically you would say this sentence without THE in English (you won't add THE unless you want to add something to nature, e.g. the nature of people). In Arabic, this sentence would be أحب الطبيعة (uHibbu-TTabí3ah) or word to word (uHibbu al-Tabí3ah). The word for nature has to come definite in Arabic الطبيعة. Notice in English, as well as THE sounds unnatural in such sentence, also adding "A" sounds unnatural as well (i love a nature) and (a swimming is a good thing). Because such nouns are not really things to be counted for real (a/an in English is really and originally "one"). Arabic on the other hand, defines the nouns for totally different purposes and majorly depends on the nature of the noun whether it is something identified by the speaker and the listener or being a well-known subject, or not.