Here are some possible interpretations of what you might have meant:
1. "You knew Spanish": The correct way to say this in Spanish would be "Tú sabías español".
2. "What did you know about Spanish?" This would be "¿Qué sabías sobre español?"
3. "That you knew Spanish": This would be "Que tú sabías español".
To help me understand what you were trying to say, please provide more context or the full sentence you saw.