Learn French: Tu sais nager ? - Do you know how to swim?

In English, we ask 'Can you swim?', but French distinguishes between learned skills and physical ability. For learned skills like swimming, driving, or speaking a language, you must use savoir (to know). You only use pouvoir for physical possibilities, opportunities, or permission.

What happens if I actually say Tu peux nager ?

It changes the meaning! Using pouvoir here asks if you have the opportunity or physical capability to swim right now. For example, it would mean 'Are you allowed to swim in this pool?' or 'Is your broken arm healed enough so you can swim?'

Where is the word for 'how' in this sentence?

French doesn't use a word for 'how' in this structure. When you use savoir with another verb to express a skill, you just put the second verb directly in its unconjugated dictionary form (the infinitive). So Tu sais nager literally translates to 'You know to swim'.

Why isn't there an est-ce que at the beginning of this question?

In everyday spoken French, it is perfectly normal to ask a question by keeping the standard statement word order and simply raising your voice at the end. You could say Est-ce que tu sais nager ?, but the shorter version in this example is very common in casual speech.