"Sounds like a great fun." Is an incorrect sentence. "Sounds like fun!" Is a better expression or, "It sounds like great fun." "Fish goes bad easily." This is the correct way to say it.

1. "Fun" is an uncountable noun and thus cannot use "a", which means "one". So, just "fun". Have some fun. It was fun. That was no fun. That was such fun. 2. "Fish goes bad easily" is okay. You can also say "Fish spoils easily".

1. You are right about this one. 'Fun' is always uncountable, so you can't have an 'a' before it. The correct sentence is 'Sounds like great fun'. NB This is an informal response rather than a full sentence - the missing subject 'It' is understood. 2. 'Fish go bad easily' is fine, because 'fish' can be a countable noun. When countable, the plural form of 'fish' is identical to the singular. For example 'How many fish are there in the tank?' 'There are ten fish'. This is obviously using 'fish' as a countable noun, referring to numbers of individual creatures. The noun 'sheep' is also invariable - one sheep/ ten sheep. So, either 'Fish go bad easily' or 'Fish goes bad easily' is correct, depending on whether you are thinking of individual objects or a general foodstuff/commodity.