I thought that you had to change 'La' to 'El' even if its feminine if the word starts with "a" (e.g. el agua)...
This is pronounced actitud, not * actitud*. So, yeah, only if it starts with a stressed "a".
You change "La" to "El" for a feminine word that starts with an "A" if the stress is on that "A". There's always an exception to the rule.
I thought that with an abstract idea, the article did not have to be translated into English.
Not when it is at the beginning of a sentence or if it is just an example phrase like this.
Si se me pregunta por actitud, y la respuesta es acttitude, mood, es una respuesta tramposa.
Por favor, por qué es "actitud" femenino? Pensaba que es masculino
Most Spanish words ending in "-ud" are feminine. See: