Learn Polish: Winogrona są drogie. - Grapes are expensive.

It is plural. The singular form is winogrono (a grape), which is a neuter noun. To make neuter nouns plural, the -o changes to an -a, just like jabłko becomes jabłka (apples).

Why does the adjective drogie end in -ie?

Because it has to match the plural noun winogrona. In Polish, plural words that don't refer to male people take adjectives ending in -e. Since the base word drogi (expensive) has a g before the ending, an i is added to keep the pronunciation soft, making it drogie.

Why do we use są instead of to in this sentence?

We use są (are) because we are describing the grapes with an adjective (drogie). You only use to when linking a noun to another noun, for example: Winogrona to owoce (Grapes are fruits).

If I wanted to say "The grape is expensive", how would that change the sentence?

You would use the singular neuter forms: Winogrono jest drogie. Notice that the adjective drogie happens to look exactly the same in both its singular neuter and plural forms!