"Mare" is a third declension neuter singular noun. Thus it requires the neuter nominative singular adjective form, "placidum."

"Cornu" is a neuter fourth declension noun. It appears in the nominative singular case in this sentence, meaning it requires the neuter nominative singular form "longum."

Although "agricola" is a first declension noun, it is a rare exception to this declension in that it is masculine, meaning it would require the second declension masculine adjective form "laetus" in order to match grammtically in case, number and gender.