Esprime is a verb form of esprimere, here meaning "to couch, to say indirectly" but more directly translated as "to express". Duo's hints here do not refer to this exercise and are misleading.

I thought it was divani and went with one of the suggestions as I thought I was wrong. This is one of the weak spots of Duolingo. I've left this site many times because they won't fix errors like these.

I agree, AliMargot. Why give us suggested words, not including divani?!

Typically words with a stress on the last letter (parole tronche) are invariable, e.g. tribù (tribe/tribes), proprietà (property/properties), tè (tea/teas). So the plural of "il sofà" is "i sofà".

"Sulli" non esiste in italiano

I gatti are all male or mixed gender cats. Le gatte would only work if all of the cats are female. So yes, just like le ragazze.

Translation: "I gatti sono sui divani." (Not sure why you're asking why not this--this is the DL translation provided)?

if it is IN (nei) the couch, not ON (sui) it, it is probably destroying the stuffing, as mine did!

Because any cat is on top of the couch, so that calls for the most literal meaning of "su" :)

Because the cats are on the couchES, so the plural form sui is required.

I am not really sure what you are asking, but Italian nouns have gender and divano is masculine. There is also an Italian word sofà which is also masculine .

http://www.wordreference.com/iten/sofa

Not before s. Before masculine words beginning with s plus a consonant. Thus la strada becomes le strade, but lo stivalo become gli stivali