poder vs saber (can)
Both translate as 'can' — but poder is physical ability or permission, saber is knowing how.
poder
Can in the sense of being able to, being allowed to, or it being possible.
No puedo venir mañana.
I can't come tomorrow.
Circumstantial — you're busy, unwell, not permitted. Not about skill.
¿Se puede fumar aquí?
Can you smoke here? / Is smoking allowed?
Impersonal poder for permission. Very natural way to ask if something is allowed.
Puede ser.
Could be. / Maybe.
One of the most useful phrases in Spanish. Possibility, not ability.
saber
Can in the sense of knowing how — a skill you've learned.
No sé nadar.
I can't swim.
You never learned. Poder here would mean you're physically prevented from swimming right now.
¿Sabes conducir?
Can you drive? / Do you know how to drive?
Skill. Poder conduces would mean 'are you allowed to drive' or 'are you in a state to drive.'
De pequeña sabía tocar el piano.
When she was little she could play piano.
Past ability/skill. Podía here would imply she was allowed or physically capable — different meaning.
Tricky bit
'¿Puedes tocar el piano?' = Are you able to right now / are you allowed to? '¿Sabes tocar el piano?' = Do you know how? The question you mean to ask is almost always saber.