"dag" cannot be singular to mean the material in general?
In Hebrew the material in general is expressed by the plural form, דגים.
I disagree. דג can mean a general plate of fish
Maybe, I'm not sure what you mean. Can you give an example? You wouldn't say הכלב אוכל בעיקר דג, would you?
Sure I would. I would rather say that than דגים there.
Interesting... it is marking that as wrong. Good to know though.
isn't mainly בדרך כלל
Sounds like "usually" to me.