1. It was a clear and sunny weather. The weather was sunny and clear. 2. It went to a corn field and saw the corns were big and plentiful. He/She/They/We/I went to a cornfield and saw the corn was tall and plentiful. Countable nouns are pretty difficult for non-native speakers. Even native speakers don't always understand why they are written a certain way. It is one of those grammar lessons we pick up naturally over time. With enough practice, you will start to notice when certain countable nouns just don't "sound right" or "look right" to you. I'm sure someone on here can give you a good definition of how to use countable nouns, but you might end up more confused haha.

Corn (the vegetable) is never plural