"The good candy."
Translation:An suiteas math.
Because suiteas is a masculine noun. So,
- Tha an suiteas math - The sweets are good. (an suiteas is nominative/accusative case)
- Tha blas math air an t-suiteas - The sweets taste good (Literally, there is a good taste on the sweets. This uses the dative case since an t-suiteas follows the preposition air)
For whatever reason, although the English word sweets is plural, its Gaelic equivalent, suiteas is singular.
This contrasts with feminine nouns, such as an t-sìde (The weather), where the t- prefix is required after the definite article for singular nouns beginning with s followed by a vowel in both the nominative/accusative case and the dative case.
Gender of a noun that is in a sentence can often be identified by how it affects words linked to it (e.g. lenition of adjectives with feminine nouns), the form of the definite article used with it, and how the definite article affects the form of the noun. See Duolingo's tips and notes for the Scottish Gaelic course for grammatical information related to this.
Without the context of a sentence, it is more difficult, and it is often just a case of memorising the gender, but see a discussion related to this at forum.duolingo.com/comment/37017991/Lenition-in-Scottish-Gaelic and links to other articles in that discussion.