To answer your first question, it depends whether the noun is definite (specific things that we know about) or indefinite (things in general). The article system in English is very simple: Definite nouns, whether singular or plural, have the article 'the'. Singular indefinite nouns have the article 'a' or 'an', depending on whether they begin with a consonant or vowel sound. Plural indefinite nouns have no article. Singular Definite = The apple Plural Definite = The apples Singular Indefinite = An apple Plural Indefinite = Apples For example, you would say: 'Apples are good for you' without an article, because you are talking about apples in general. but 'The apples on that market stall are really good' because you are talking about certain specific apples. And to answer your second question, you say 'the water' because it's the water which comes out of the taps and plumbing system in that house. It's not 'water' in general.

Yes, "the" can be used with plural nouns. There is no separate form of "the" that is used with plural nouns. We are talking about "the" water specifically in and for that bathroom, not water in general, so you must use "the". You couldn't have random water magically come out of your bathroom faucet.

Unlike most Romance languages (Spanish, Italian French,) English does not have different words for the article "the" in singular and plural. It is the class - singular Or, it is the classes - plural. They are THE lessons. It is THE lesson.

The specific questions that we're talking about are difficult, not questions in general. At far as the bathroom, we are referring to the water supply that is in the bathroom that we already know about, not some water supply that just magically appeared out of nothing. I hope my explanation helps a little.