"No, one lemon is not good."
Translation:Nein, eine Zitrone ist nicht gut.
First, ‘Zitrone’ is feminine, whereas ‘eins’ is neuter. Second, before a noun such as ‘Zitrone’ you need an article, whereas ‘eins’ is a pronoun.
If you substitute a neuter noun, such as ‘Bier’, you get ‘Nein, ein Bier ist nicht gut.’ Replace the neuter noun phrase ‘ein Bier’ with a pronoun, and you get ‘Nein, ins ist nicht gut.’ But replace the feminine noun phrase ‘eine Zitrone’ with a pronoun, and you get ‘Neine, eine ist niche gut.’
Eins (one) as in "one thing" conjugates. So ein can mean either 'one' or 'a/an', which you have to determine by context. But usually ein/eine translates to 'a/an'.
You'll see with this sentence, that both "one lemon is not good" and "a lemon is not good" mean very much the same thing.
Hmm… Good question. The ‘nicht’ can't occur after a predicate adjective such as ‘ist gut’ or a predicative nominative such as ‘ist König’. But invert the equational sentence and it can: ‘Gut ist eine Zitrone nicht.’ = “Good is not what a lemon is.”; ‘Köng ist er nicht.’ = “King he is not.”.
Correct. And when an ordinary verb has a complement, the ‘nicht’ occurs either immediately after the verb and before the complement, as in ‘Ich esse nicht die schlechte Zitrone.’ = “I'm not eating the bad lemon.”, or after the whole verb phrase, as in ‘Ich esse die schlechte Zitrone nicht.’. Without emphatic stress, the former would more readily be understood to negate the eating (as opposed, say, to throwing it out); and the latter, the bad lemon (as opposed, say, to a good lemon).