The verb essere (sono, sei, è, siamo, siete, sono) is usually used for permanent conditions. The verb stare (sto, stai, sta, stiamo, state, stanno) is usually used in temporary/non-permanant situations.
Essere literally translates as "to be", and stare literally translates as "to stay", however Italian sometimes uses stare where English would use the verb "to be". This can be confusing for beginners, but it's just one of those things you have to learn as part of learning the language.
This link might also help: http://serenaitalian.wordpress.com/2010/09/09/difference-between-stare-and-essere/
I probably don't have this completely right, but I'll take a shot. 'Nel' is used with most masculine singular nouns, the ones that use the article 'il'. 'Nello' is used with the masculine singular nouns that use the article 'lo'. I think they are the ones that begin with the letter s plus a consonant, the letter z, or a vowel. I think there are others as well. Because you say 'il piatto', not 'lo piatto', this example uses 'nel'. :)