Why is it sú oráiste and not otáiste sú?
Adjectives are put after nouns in Irish.
When do you use "toradh" as opposed to the other spelling of fruit?
toradh is one fruit, torthaí is plural
Is there a rule for when to use an article, and when not to?
The rule is simple, there is no indefinate article in Gaeilge. Definate articles are used as in English.