Learn Swedish: De stora grupperna samlas på torget. - The large groups are gathering in the square.

It looks like a passive verb, but here it is an s-verb with an active, reciprocal meaning. While samla means to collect items, samlas means to gather together with each other. This is similar to how ses means to see each other.

Why is it De stora grupperna instead of just Stora grupperna?

Swedish uses double definiteness when an adjective describes a definite noun. You need the independent plural article De (the), the adjective with its definite ending (stora), and the definite suffix attached to the noun itself (grupperna).

Why do we use the preposition på when the English translation says in the square?

In Swedish, open public spaces like squares (torg) or streets (gator) take the preposition på (on). This is because you are physically on the flat surface of the open space, rather than enclosed inside a building.

How is the starting word De pronounced here?

Just like when it means they, the plural definite article De is almost always pronounced like the word dom. When speaking naturally, you would say dom stora grupperna.