Learn Italian: La penisola è pianeggiante, ma la Grecia meridionale è montuosa. - The peninsula is flat, but southern Greece is mountainous.

As the subject of this sentence, Grecia takes the definite article la regardless. However, your current lesson highlights a broader rule: because the country is modified by an adjective (meridionale), it is now locked into keeping its definite article in all grammatical contexts. For instance, if you used the preposition in, "in Greece" is just in Grecia (no article), but "in southern Greece" forces you to keep the article and combine it with the preposition: nella Grecia meridionale.

Could I say la Grecia del sud instead of la Grecia meridionale?

Yes, absolutely. Meridionale is a specific geographic adjective for "southern." Using the specification del sud (of the south) is also perfectly correct and very common in everyday conversation. Based on the same article retention rule, la Grecia del sud also keeps its definite article in all contexts because the country is being specifically modified.

Why does pianeggiante end in -e when it describes the feminine noun penisola?

Pianeggiante (flat or level) belongs to the group of adjectives that end in -e in their singular dictionary form. These adjectives use the exact same ending for both masculine and feminine singular nouns. This is unlike montuosa, which changes its ending to match the feminine Grecia because its base form is the four-ending adjective montuoso.