It seems that my comment got cut off - no wonder really... So, it will be 'to je cveće' - because the form of cveće is that of a singular neuter noun, even though it actually refers to plural. 2) I think that I've already answered this question, but I'll try and explain again: We use 'je' with nouns in singular form, and 'su' with nouns in plural form. Zgrada is a common noun with a regular plural form, so we'll say 'To su zgrade'. Cveće, as I've already said, is a collective noun which has a singular form, so we'll say: 'To je cveće', even though we are talking about many flowers. 3) We create short answers with conjunctions such as 'nego' and 'već': 'To nije zgrada, nego kuća' / 'To nije zgrada, već kuća'. Yes, 'To nije zgrada, je kuća' would be incorrect. I hope I helped and I hope I didn't confuse you even more... If you have any other questions, I'd be happy to help you :)

1) Actually, there are rules for building plural of nouns. The plural (nominative) of feminine nouns (they end with -a in nominative, e.g. kuća, tašna, stolica, žena) is formed with -e (kuće, tašne, stolice, žene). The plural of masculine nouns (they usually end with a consonant, e.g. papir, novčanik, avion, zid, put) is a bit more complicated. For some nouns, the plural (nominative) is formed by simply adding an -i: papiri, avioni, novčanici. You mentioned novčanici yourself - the ending is -i, but there is also a sound change in this case, where K, G, H in front of I change into C, Z and S, respectively. This most notably happens in declension (though there are exceptions), so it will be novčanici (nom. pl.), novčanicima (dative/instrumental/locative plural). Then there is another group of masculine nouns, where plural is formed by adding an infix (-ov- or -ev-) before the ending -i (and I'm afraid that you will have to learn by heart which nouns have the infix, and which don't). In that case, it will be zid + ov + i = zidovi; put + ev + i = putevi. There are rules which regulate whether the infix will be -ov- or -ev-, but there is also a number of exceptions. The plural of neuter nouns (they end with -o or -e, e.g. selo, polje, ime, pleme) is formed with the ending -a, and it's perhaps easiest to rembember that the nominative plural of neuter nouns is the same as the genitive singular: selo - sela, polje - polja; but: ime - imena, pleme - plemena. And finally, the irregular plural - the most common word with the irregular plural is dete, where the nominative plural is deca (and while dete is a singular neuter noun, the form of deca is actually that of a singular feminine noun - crazy, right?). Cveće and drveće (that you mentioned) are actually collective nouns; that means that they denote multiple objects (as if it were plural), but their form is singular (so it will be 'to je cveće' - because the form of cveće is that of a singular neuter noun, even though it a