Learn Italian: Il bambino sa nuotare molto bene.

The verb sapere usually means "to know", but when it is followed directly by an infinitive verb (like nuotare, meaning "to swim"), it translates to "knowing how to" do something. In English, we usually just say "can" for this learned skill. So sa nuotare means "knows how to swim" or "can swim".

Could I use può (from potere) instead of sa to say "can"?

In Italian, there is an important difference! You use sapere for learned skills, like knowing how to swim, speak a language, or ride a bike. You use potere for permission or physical possibility, like asking "can you swim today, or is the pool closed?". Since the child has learned the skill of swimming, we use sa.

Why is nuotare in its full dictionary form (the infinitive) instead of being conjugated?

Whenever you have a conjugated verb like sa directly followed by another action, the second verb stays in the infinitive form (ending in -are, -ere, or -ire). You only conjugate the first verb.

Why do we use bene here instead of buono?