El águila vuela muy alto.

Because águila is a feminine noun that begins with a stressed a sound. In Spanish, singular feminine nouns like this usually take el instead of la to make pronunciation smoother.

So you say:

el águila
not la águila

But the noun is still feminine. In the plural, the problem disappears, so it becomes:

las águilas

Yes, águila is still feminine.

The change to el only affects the singular article form in this special pattern. Other words normally still show feminine agreement:

el águila blanca
un águila herida
esta águila

So the noun is feminine even though it uses el in the singular.

The accent mark shows that the stress falls on the first syllable:

Á-gui-la

Without the accent, normal Spanish stress rules would make it sound like a-GUI-la, because the word ends in a vowel. That would be incorrect, so the accent mark is necessary.

A good English approximation is AH-gee-la.

A few useful details:

• The stress is on the first syllable: Á
• In gui, the u is silent
• So it is not AG-wila

For a learner, AH-gee-la is a helpful approximation.

Vuela is the third-person singular present indicative form of volar, meaning to fly.

So:

• yo vuelo
• tú vuelas
• él/ella vuela

This verb is stem-changing in the present tense:

o → ue

So volar becomes vuela, not vola.

A good approximation is BWEH-la.

Two key points:

• The v in Spanish is pronounced much more like b than like the English v
• ue forms one glide-like sound

So it does not sound like English vee-ella.

Because muy is the word used before adjectives and adverbs to mean very.

Here, alto means high, so:

muy alto = very high

By contrast, mucho usually means a lot, much, or many:

vuela mucho = it flies a lot
muchas águilas = many eagles

So muy alto is the correct combination here.

Here alto is not directly describing the noun águila. It is describing how the eagle flies, so it works adverbially.

That is why it stays in the basic masculine singular form:

El águila vuela alto.
Las águilas vuelan alto.

Not altas.

This is common after certain verbs in Spanish, especially with words like alto, bajo, claro, and similar expressions.

In Spanish, el + singular noun can be used to talk about a species or a whole class of animals in a general way.

So:

El águila vuela muy alto

can be understood as a general statement about eagles as a kind of animal, not only one specific eagle.

By contrast:

Un águila vuela muy alto

sounds more like an eagle flies very high or one eagle flies very high, referring to an unspecified individual.

Normally, no.

In standard Spanish, a singular count noun used as the subject usually needs an article or another determiner:

el águila
un águila
esta águila

So Águila vuela muy alto would sound unnatural in normal speech. You might only see something like that in very special styles, such as headlines or labels.