This rules only applies to Spanish words where the beginning of the word is a stressed á. El agua and un agua are used to prevent word chaining, which is "L (aa) gua" Where vowels chain therefore in a sense you skip the a in agua and say gua which sounds really strange. Go ahead and say "Laa Gua" with a Spanish accent. If you still confused or don't see a difference then work on that accent my friend. Here I would have wrote this sentence as "Cuántas águas necesitas" o "Cuánta água necesitas" Where either I break the word chain or when written with a stress tick indicating the break. When spoken, you would not see this, but written as it is really does get to me.
"Necesitas" is singular second person familiar, so "you" must be included as the subject of the sentence: How much water do you need?/¿Cuánta agua necesitas? If the sentence had been third person singular, "¿Cuánta agua necesita?", then it could have been translated as singular "you" formal: "How much water do you need" or with the null subject (it)" "How much water is necessary?" The reason why is because "necesitar" means "to need." "Is necessary" and "is needed" are interchangeable in some English sentences, but not in all of them.
The difference is between active and passive voice. Active voice has a subject (a person doing something) while passive voice does not - it just states what is being done.
The phrase "How much water is needed" uses the passive voice, and is translated as "¿Cuánta agua se necesita?" or "¿Cuánta agua es necesitada?" Both of these are valid ways to say it, but I would use the first option ("se necesita"). (Note, I'm not a native speaker, and the preferred way to say it may vary from place to place; I'm not sure.)
I would personally translate "How much water is necessary" as "¿Cuánta agua es necesaria?", which also uses the passive voice.
"¿Cuánta agua necesitas?" has an implied subject (i.e. "¿Cuánta agua necesitas tú?"), and thus is in the active voice: it has a subject ("You") and the action that subject is doing ("need").
Even though the stressed first syllable demands masculine articles, the noun itself is still feminine and takes feminine gender adjectives. Thus, "cuánta agua" (cuanto/a can be an adjective or an adverb) and "el arma pesada" (the heavy weapon). Also, feminine gender nouns that for one reason or another use the masculine article when their number is singular still use the feminine article when their number is plural, as in "las armas pesadas."
Well explained again Linda from NJ. I now understand. Toma otro lingot.
agua fria (feminine adjective...because feminine word) El agua (but masculine singular article i.e el even though femine word) Las aguas de Babilonia (but feminine plural article i.e las)
I think the above is correct but do please correct me if I am wrong.