Learn Russian: У строителя есть молоток, но ему нужен гвоздь. - The builder has a hammer, but he needs a nail.

In Russian, expressing possession and expressing need require two different grammatical structures. To say "someone has," you use у + Genitive case (у строителя). To say "someone needs," you put the person in the Dative case (ему, meaning "to him").

Why is нужен in the masculine form?

The word нужен is a short adjective meaning "needed," and it always agrees with the thing being needed, not the person who needs it. It is masculine here because it agrees with гвоздь (a nail), which is a masculine noun. If the builder needed a feminine object like a board (доска), you would say ему нужна доска.

What case is гвоздь in?

It is in the Nominative case. In this "need" construction, the thing you need is actually the grammatical subject of the sentence. A literal translation of ему нужен гвоздь is "to him a nail is needed."

Since гвоздь ends in a soft sign (-ь), how do I know it's masculine?

Nouns ending in a soft sign can be either masculine or feminine and usually just have to be memorized. However, this sentence gives you a huge clue! Because it uses the masculine adjective нужен rather than the feminine нужна, you can immediately tell that гвоздь is a masculine noun.