"This year was a good year."
Can someone explain me when it is pronounced as nen? and when as toshi? Thanks.
I'm pretty sure nen is used as the counter for year years eg. 三年 = san nen = 3 years, whereas toshi is referring to the year in general, if that makes sense
Can one use the "~katta" conjugation when the noun is expressed? ex: "yokatta nen desu" instead of "ii nen deshita"
No; in that case, you'd conjugate the verb/copula at the end rather than the adjective. The adjective only gets conjugated to past tense when it's not prenominal (i.e. before a noun).
I don't believe you can, but I can't really explain why. Hopefully someone else can.