Το «α» είναι φωνήεν.

Greek usually uses the definite article with letters when naming them. So το α literally looks like the a, but idiomatically it means the letter a.

Here, το is neuter singular. Letter names in Greek are commonly treated as neuter.

Those are Greek quotation marks, often called guillemets. They show that α is being mentioned as a symbol, not used as part of a normal word.

So «α» means something like the character a. In modern typing, you may also see ordinary quotation marks instead.

The letter α represents the sound /a/, like the a in father.

But if you are reading the sentence aloud, you would normally say the name of the letter: άλφα. So aloud, many speakers would read the sentence as Το άλφα είναι φωνήεν.

Είναι means is. It is the 3rd-person singular form of είμαι, which means to be.

It is pronounced roughly EE-neh, more exactly /ˈine/. The stress is on the first syllable.

After είναι, Greek often uses a bare noun when classifying something. So είναι φωνήεν is the normal way to say is a vowel.

You can sometimes hear είναι ένα φωνήεν, but the version without ένα is the more neutral, textbook-style way to define or classify something.

It is pronounced roughly fo-NEE-en, or /foˈni.en/.

The stress falls on νή. Also, the last part is pronounced in two syllables, so it is φω-νή-εν, not one merged ending.

In Modern Greek, words of more than one syllable normally show the stressed syllable with an accent mark. That is why είναι and φωνήεν have accents.

Monosyllabic words like το and α usually do not need one.

Yes, absolutely. That version uses the full name of the letter instead of the symbol.

In speech, Το άλφα είναι φωνήεν is often even more natural, because people usually say the letter name aloud rather than just naming the written character.

It can mean either, depending on context.

In this sentence, the subject is the written letter α, so φωνήεν means a vowel letter. In phonetics or pronunciation contexts, φωνήεν can also mean a vowel sound.

They are historically related and look similar, but you should not think of them as behaving exactly the same way.

Greek α has a stable sound /a/, like the a in father. English A is much less predictable, and its letter name is ay, which is not how Greek α is pronounced in words.