Fish swim freely in the vast ocean.

Meaning: This sentence means that fish are able to move around easily in the big ocean.

Choice of Words: Fish are animals that live in water and have fins. Swim is a verb that means to move through water using your fins. Freely means without any restrictions or limits. Vast means very large in size or amount. Ocean is a large body of salt water.

Alternative Expressions

The fish are swimming without any restrictions in the huge ocean.

Meaning: This means that the fish are able to swim wherever they want in the ocean, and there's nothing stopping them.

Choice of Words: The fish are still the subject, but the are swimming is a different verb form that emphasizes the ongoing action. The without any restrictions tells us that the fish are free to swim as they please, and the huge ocean gives us a sense of the size of the area where they're swimming.

In the boundless expanse of the sea, the aquatic creatures engage in the act of swimming unhindered.

Meaning: This means that the fish are able to swim anywhere they want in the ocean, and there's nothing to stop them.

Choice of Words: The aquatic creatures is a more formal way of referring to the fish, and the engage in the act of swimming is a more elaborate way of saying swim. The boundless expanse and the unhindered both emphasize the vastness of the ocean and the freedom of the fish to swim in it.

Related Expressions