Although “a lot” is, grammatically speaking, a noun in the singular (like “a house” or “a book”), “a lot of” functions as a so-called quantifier. Quantifiers are used to express quantity or a number of objects; here are some of the most common:
Some languages (e.g. most Slavic languages) assign a number to the quantifiers themselves, and it is this number that determines the correct verb form (the quantifier itself becomes the subject of the sentence where it is present). English, however, works in a different way.
Quantifiers function as adjectives in English, no matter what their inner structure is, and they are never considered the subject of the sentence. The correct verb form is determined by the real subject of the sentence, i.e. by the noun or pronoun modified by the quantifier.
This is best illustrated using the quantifier “some”, which almost all learners quickly learn to use correctly:
Some water is being wasted right now.
Problems seem to arise when a quantifier that looks like a noun is used, such as “a lot of”, which looks like a singular noun but is not:
or “lots of”, which looks like a plural noun but is not:
Note that many and (a) few may also be used as plural pronouns, rather than quantifiers, as in the following quotation from the English version of the Bible:
Similarly, much, (a) little, and a lot may be used as singular pronouns:
Finally, all and some may be used as either singular or plural pronouns, depending on whether they represent a mass noun or a countable noun:
All have turned away.