Learn German: Das Geschenk ist vom Experten. - The gift is from the expert.

The noun der Experte belongs to a special group of masculine nouns called weak nouns (or N-declension). These nouns add an -n or -en in every case except the nominative (the subject). Because it follows a dative preposition here, Experte becomes Experten even though there is only one expert.

Why do we use the dative case here?

The word vom is a contraction of the preposition von (from) and the article dem (the). The preposition von is a dative preposition, meaning the noun that follows it must always be in the dative case.

If Experten is the singular dative form, how would I say from the experts in the plural?

You would say von den Experten. The weak noun Experte looks exactly the same in the plural as it does in the singular dative (Experten). To tell if it is one expert or multiple, you have to look at the article: vom (von + dem) means it is singular, while von den means it is plural.

What if the expert is female? Do I still use the N-declension?

No, N-declension only applies to masculine nouns. A female expert is die Expertin. In the dative case, this would just be von der Expertin. Female nouns do not get the extra weak noun ending.