The word nadabas is the imperfect form of nadar in the second person singular. There are other translations for this conjugation.
See the full nadar conjugation

nadar

Lo sé, pero nunca nadabas en lo profundo.
I know, but you never even used to go in the deep end.
Pensé que no nadabas.
I thought you didn't swim.
En esas o nadabas o te ibas al fondo, que estaba muy profundo.
In those pools, you either swam or you sank deep down to the bottom.
No sabía que nadabas.
I didn't know you swim.
No sabía que nadabas tan bien.
I didn't think you could swim.
El año pasado yo tenía frío en Italia mientras que aquí tú nadabas, ¿verdad?
Last year when I was in Italy it was freezing and here you were swimming, right?
Sueño contigo a menudo. Anoche soñé que nadabas con ballenas.
I dream of you often. Last night I dreamed that you were swimming with whales.
¡Esta alberca es enorme! ¿Nadabas aquí de joven?
This pool is huge! Did you swim here growing up?
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