когда ставят предлог " a " а когда "an"

a - пишется когда слово начинается с согласный букв. an- когда слово всегда начинается гласным. Например: 1) a toy was lying on the floor..... a dress which costs $50.2) An Elephant was taking shower with its own trunk... an imergency help was needed to the lost animals which got injured.