Translation:My brother swims in the river every morning.
How and why does râu (river) become fluviu (the river)? They don't share any similarities as other declensions do - they don't even have the same stem! Are there any other words like this in Romanian?
They might have the same translation in English but they mean different things in Romanian. " fluviu" is a big river that ends in an ocean or in a sea, and "râu " is a smaller river that ends in a "fluviu". The same situation is in French ("fleuve " , "rivière ") as this comes from Latin ("fluvius ", "rivus ")