HERE AM I= This is not a question, so we shouldn’t have an inversion. In fact, we can say "here I am", but in old English, when a sentence begins with an adverbial there was always an inversion in the order of subject and verb. In modern English that kind of inversion only happens with some adverbials (for example: "not only does she speak French but also German"). Anyway, with some other adverbials, like this one, we don’t use inversion now, but if we do, it sounds archaic and poetical, so Enya is using the inversion in this opening sentence to make it sound more beautiful and old-fashioned.

YET ANOTHER= One more. We could just say "another", but adding "yet" makes it more emphatic. So this phrase suggests that there were many other goodbyes before this one.

SAYS= Remember that the form SAY is pronounced /seɪ/ but the form SAYS is pronounced /sez/.

ADIOS= This is a Spanish word that means "good-bye". There is an English word related to this one, of a French origin: "adieu" (pronounced /ədju:/), but that word is only used when you know you’ll never see each other again (because one person is going to die, or travel to a distant country never to return, etc).

BREAK DOWN= If a machine breaks down, it stops working because there is a problem. If a person breaks down, they have an emotional collapse, so they may fall to the ground and cry or shout. To have a nervous break-down is even more serious, the person suddenly feels so sad and desperate that they can’t control their emotions or their actions. Although breaking down usually implies crying, it is very common to use the phrase BREAK DOWN AND CRY.

ONE BY ONE= This phrase means that something is happening one at a time, slowly but nonstop. So first one leaf falls, then another one, then another one, etc. And very probably all the leaves will fall eventually.

LEAVES= Plural of LEAF. The leaves of a tree are the little green parts that the plant uses to breathe and make chlorophyll. Although we prefer the word "pages", you can also use the word LEAF to talk about the sheets of a book. In this song, this word has both meanings. The leaves fall just like a tree in autumn, getting cold, sad and bare. But the leaves of her book of life tell the story of her love failures ("my tales are told").

IT’S NO LIE= The negative article NO is usually replacing NOT ANY, so we use it for uncountable and plural nouns. But we can also use NO for singular countable nouns replacing NOT A, like here (IT’S NO LIE = IT’S NOT A LIE). This use of NO is more common in colloquial English but it’s fine, and more emphatic than the equivalent with "any".

YEARING= If you yearn for something you feel melancholic and desire it intensely.

HE’S A REASON TO SIGH= This is the contraction of HE HAS. This contraction is common when we use the form HAVE GOT (he’s got a reason). When we use the form HAVE/HAS (without GOT) to express possession, we don’t usually contract the verb, but it is possible to do it. In this song, she preferred the contracted form to match the music speed.

SIGH= To make a sound by just breathing a lot of air out. We often make this when we are tired, relieved or very sad. She feels trapped in this relationship and says goodbye, so he feels very sad, that’s why the song says: "and now you know why he’s a reason to sigh".

MY, OH MY!= This is an exclamation to express surprise, sadness, joy, worry or many other things. The complete expression is "Oh, my God!", but we can also say "Oh, my!" or simply "My!". You can also hear sometimes this combination: "my, oh my!"

SHE’S AIMING TOO HIGH= "To aim" is to direct to an objective. For example, before shooting an arrow you aim at your objective, because you want to send your arrow there. So if she’s aiming too high, it means that her objective is impossible to get. If you want to shoot an arrow to the moon, you will never succeed, because that objective is too far for your capabilities. This sentence means that she’s looking for something impossible. She probably has an unreal image of love and wants a lover who is perfect in every way, but if you are looking for perfection, no relationship will succeed, because perfection is impossible to get, so all you will get is another goodbye.

THERE’S NO MOON IN HER SKY= Again, we have NO with a singular noun (= there isn’t a moon). The moon is a symbol of romanticism, and it is also the only light you have in the sky at night, so no moon in her sky means that she is living in the dark (sadness) without hope because there’s no love and there won’t ever be.

BRIGHTEN= If something is bright it is shining, it has light. So the verb TO BRIGHTEN is to illuminate, to bring light to a place. If your eyes brighten, you are happy and smiling.

FOR LOVE BRIGHTENS THEIR EYES= In this sentence FOR means BECAUSE.