To remember easily, remember this:

Nominative = Subject

Accusative = Object

So when you say “Mama loves baby.”

Mama = Nominative (subject)

baby = Accusative (object)

Mama is “mama” in Serbian and baby is “beba”. For nominative, you will just write the original word without changing anything, so here mama will remain mama, but for accusative, you will change it according to the gender of the word and if it is alive or not. 

So here, the translation will be:

“Koza jede travu.” 

As you notice, “Koza” remains koza because it’s nominative, it remains the same but trava became travu. Why is that? Because as I mentioned above, accusative is changing according to the gender of the word. 

Accusative rules are on the table: