"I used to know how to swim."
Translation:Yo sabía nadar.
saber + infinitive = to know how to do something and no 'como' is added. The como was not dropped --it was never there. ;)
Sí, it's the same in Italian, French, and Portuguese. It's just the way the romance languages do it.
"used to" should not be used in the English in this case. "Used to" can be properly used to translate the Spanish imperfect when it is used to communicate a habitual ACTION in the past. Knowing how to do something is not an ACTION, it is a state or capability.
Adding "used to" in English when you are talking about a state means that the state no longer exists. The Spanish sentence does not say that he no longer knows how to swim. Since the imperfect is used, the Spanish is actually silent on this point. It doesn't say anything other than at the point were are talking about, he knew how to swim. Maybe he is dead now, maybe he is alive and can't swim. The Spanish doesn't care about any of this, it is just making a statement about his capability at this point in time.