Learn Russian: Я заплатил за кофе пятью монетами.

In Russian, the verb заплатить (to pay) pairs with the preposition за plus the accusative case to indicate what you are paying for. You can't just use the noun directly after the verb to mean you paid for it.

Is кофе in the accusative case here?

Yes. It follows заплатить за, which requires the accusative. However, because кофе is a foreign borrowed word, it is indeclinable, meaning its spelling never changes no matter what case it is in.

What form is the word пятью?

Пятью is the instrumental form of the number пять (five). It is used here to show the instrument or method of the action, meaning you paid 'by means of' five coins.

Why isn't the noun монетами in the genitive plural, since it follows the number five?

Normally, when a number like five is in the nominative or accusative case, the noun after it takes the genitive plural. But when the grammar of the sentence requires the phrase to be in any other case (like the instrumental here), both the number and the noun must match in that case. Therefore, both take the instrumental form: пятью монетами.