The genitive of the singular of the second declension ends in –ī.

Scapha puerī in īnsulā est = The boy’s boat is on the island.

The plural nominative of the second declension also ends in –ī, but since the way the plural nominative functions in the sentence is very different from the singular genitive of the second declension, it shouldn’t be too difficult to spot the differences between the two.

Puerī in īnsulā sunt = The boys are on the island.

Here, puerī end in an –ī, but you can clearly see that there is no possession in the sentence. Therefore it has to be the nominative plural.

The genitive plural of the second declension ends in –ōrum.

Scapha virōrum in aquā = The mens’ boat is on the water.

Be careful with the genitive plural of the second declension. It is similar to the genitive plural ending of the first declension, which ends in –ārum.

Scapha pīrātārum in nōn aquā = The pirates’ boat is not on the water.

Here, pīrātārum ends in -ārum, which tells you that this is a first declension gentive singular noun.

The genitive can also have the meaning “of”, especially when “of” refers to possession :

Incola īnsulae es = You are an inhabitant of the island. (incola = inhabitant)

Here are some exercises :

1) Incola silvae sum

2) Convīvae poētae in casam ambulant

Here are the answers :

1) I am an inhabitant of the forest.

2) The guests of the poet is walking into the house.