если and если бы

Russian has two types of condition.

• Real condition: если + future, then future. Unlike English, Russian uses the future after если: Если завтра будет дождь, мы останемся дома (If it rains tomorrow, we will stay at home).
• Unreal condition: если бы + past tense, and the result clause also has бы + past tense: Если бы я знал, я бы тебе помог (If I had known, I would have helped you).

The past form agrees with its subject: я знал, я знала, мы знали. One form covers both present and past situations: Если бы у меня были деньги, я бы купил машину means if I had money or if I had had money, depending on context. Do not drop бы in the result clause.