Leithio, leithio!

Sí avon nedh'ared han

Adlego, adlego

Avon dangened

i-phith bain ú-charnar nin

Tolo, alagos!

i-Chelch gîn ú-drasta nin.


Literal English Translation:

Let it go, let it go!

Now I won't hold it inside

Let it go, let it go!

I won't look back

No words can wound me

Come, storm!

Your cold doesn't bother me.

Even more literal translation:

Let [it] go, let [it] go,

Now will-not-I inside-hold it

Let [it] go, let [it] go,

Will-not-I behind-look

The-words all not-wound me

Come, storm!

Your cold doesn't bother me

No one is this effing literal translation:

Leithia-(v: inf) becomes leitho (v: imp) "Let [it] go"

Sí: (adv) "Here/now"

Av-(v: inf) becomes avon (V: present first person singular) "I will not"

Nedh-(prefix) "inside"

Gar-(v: inf) becomes gared (n: gerund) mutates to *'ares as it follows prefix "hold"

Adlegi-(v: inf) becomes adlego (v:imp) "Let [it] go"

Av-(v: inf) becomes avon (v: present first person singular) "I will not"

Dan-(prefix) "back-"

Cen-(v: inf) becomes cened (n: gerund) mutates to *gened as it follows prefix "look"

i: (adj: singular) becomes in(adj: plural) mutates to *i-ph as it directly precedes 'p' "the"

Peth: (n: singular) becomes pith (n: plural) nutates to *phith as it directly follows 'in' "Word"

Pen: (adj: singular) becomes pain (adj: plural) mutates to *bain because it directly follows its noun "all"

Û: (adj/adv) becomes *ú- when prefixed to verb "ill, bad, no, not (negates verb)"

Harna-(v:inf) becomes harna (v: present personless singular) becomes harnar (present personless plural) mutates to *charnar as it directly follows negation "wound"

Nin: (pron: first person singular objective) "me"

[?]Teli-/tol-(v: inf) becomes √tolo (v: imp) "come"

Alagos: (n: singular) "storm"

Helch: (n: singular) mutates to *chelch as it directly follows 'the' "cold"

Cîn: (pron: second person singular informal possessive; adj) mutates to *gîn as it directly follows the noun "your, yours"

Trasta-(v: inf) becomes trasta (v: present third person singular) mutates to *drasta as it directly follows negation "troubles, bothers"

Nin: (pron: first person singular objective) "me"

Sources:

1) Hiswelókë's Sindarin-English Dictionary: "Strict" Sindarin & Noldorin Alphabetical and Analogical Dictionaries.

2) Thorsten Renk's Pedin Edhellen, a Sindarin-Course, Version 3.05 (July 21st, 2010)

3) The wonderful advice of Tolkien scholar and linguist dreamingfifi, whose 'realelvishDOTnet' is a tremendous resource for any fan fiction writer or hobbyist translator.