Hi Jongmyeong, I summarized the info in the link @unfanatiko shared with you in english , The water noun is feminine, but since it starts with the letter "a" and in which the accent falls, due phonetic reasons (cacophony) this kind words use the singular article 'el' instead of the female form 'la'. This rule operates only when the article immediately precedes the noun,thats why we say 'el agua', 'el área', etc but when between the article and the noun stands another word, the rule is not effective, hence we say 'la misma agua' , 'la extensa área' etc. the same happends with the use of 'y' and 'e' (cacophony) and we say 'hijos e hijas' instead of 'hijos y hijas' I hope it helps. Saludos y éxito tu español :)

es el agua. No se usa "la" cuando comienza el sustantivo con esa letra, si empieza con h tampoco por no se pronuncia.

Correcto, acá está bien explicado: http://www.rae.es/consultas/el-agua-esta-agua-mucha-agua Espero te sirva.