COMMON MISTAKE When a feminine singular noun begins with a stressed "a" or "ha"
use the article "el" rather than "la." This is done simply for
pronunciation purposes. Let's look at a common example:

el agua

To understand why we use "el" rather than "la" try pronouncing
the following very rapidly three times:

la agua
la agua
la agua

Notice how the final "a" in "la" tends to blend with the
beginning "a" in "agua" with the result sounding like a different
word altogether:

lagua

So, to avoid this, we simply change the "la" to "el." Say the
following rapidly three times:

el agua
el agua
el agua

But the common mistake is for the student to believe that because
"agua" is preceded by the definite article "el" it must be
masculine. It is not! "Agua" is still feminine, even though it
uses the definite article "el." As a feminine noun, it calls for
a feminine adjective. The following would be correct:

el agua sucia (the dirty water)

Notice that even though we use the definite article "el" we do
NOT use the masculine adjective (sucio). Notice also that the
masculine definite article is only used for the SINGULAR form. In
plural, we do not have the same pronunciation problem. Say the
following rapidly three times, and notice that there is not a
pronunciation problem:

las aguas
las aguas
las aguas

Thus, each of the following is correct:

el agua (the water)
las aguas (the waters)
el �guila (the eagle)
las �guilas (the eagles)