"This curry is delicious."
If you use が, it will identify the curry you are eating at the only one delicious. You can use が, for exemple, if you never liked curry before, if the other curry in the restaurant taste bad, if you don't like what you had already eaten,...
I used とても before おいしい. I think that both is correct to translate the sentence.
とても おいしい = very/really delicious, which isn't equivalent to the English sentence which only asks for delicious.
I thought おいしい meant 'tastes good' so would とてもおいしい be 'very tastes good' or 'delicious'?
The problem here is that おいしい does not specify a degree, but "delicious" does. In English, "delicious" is already a superlative for good/tasty (delicious = very good/very tasty), and for what I understood おいしい just means that something tastes good, is tasty or is even delicious (but doesn't always mean that something is good enough to be delicious).
So if おいしい = "good"/"tasty"/"delicious", とてもおいしい = "very good"/"very tasty" and "delicious" = "very good"/"very tasty", then it makes sense that とてもおいしい = "delicious".
Then, "very delicious" has another problem: it is redundant. As delicious is already a superlative, "very delicious" is not giving any more (semantic) information than "delicious", other than emphasizing how you feel about it, since it's the same as adding another "very" to "very good". The first "very" already contemplates the possibility of it being infinitely good, but you can add more "very"s for emphasis ("It's my favorite dish. It's very very very good"). That said, "very delicious" = "very very good", so as とてもおいしい can mean the same as "very good", putting just "delicious" should be fine and correct.
Sorry for such a long comment. I got a little lost in my own explanation, not knowing how to make my point in a straightforward way
couldn't おいしい stand alone, I mean without です? 「このカレーはおいしい」 vs 「このカレーはおいしいです」?
Not sure why using が here gets this marked as incorrect. Surely I'm identifying this specific curry, to the exclusion of all others, as the delicious one?
I still don't get the difference between この and これ, could I use これ in this sentence instead?
これ is the pronoun, when you just use "this" as the actual subject of the sentence. In this one, you'd have to drop curry if you wanted to use it: これはおいしいです = "this is delicious". この is the determiner, it needs another noun following it because it doesn't stand on its own, hence "this curry" = このカレー.
これはわたしの本です。 = This is my book.
この本はおもしろいです。 = This book is interesting.
これ is a standalone pronoun; この needs to be followed by a noun.
I thought desu on the end was optional for politness, but duo telling me its wrong without it??