Latin text
The text of a requiem mass consist of several parts: Introitus: Requiem aeternam, Kyrie, Graduale: Requiem aeternam, Tractus: Absolve, Domine, Sequentia: Dies Irae, Offertorium: Domine, Jesu Christe, Sanctus + Benedictus, Agnus Dei, Lux Aeterna, Pie Jesu, Libera me and In Paradisum (not original). Here I give the original Latin text and the English translation. Please note that some composers use variations.
1. Introitus: Requiem aeternam
Requiem aeternam dona eis, Domine.
Et lux perpetua luceat eis.
Te decet hymnus, Deus, in Sion,
Et tibi reddetur votum in Jerusalem
Exaudi orationem meam
Ad te omnis caro veniet.
Eternal rest give unto them, O Lord
And let perpetual light shine upon them
A hymn, O God, becometh Thee in Zion
And a vow shall be paid to thee in Jerusalem
Hear my prayer
All flesh shall come before you.
2. Kyrie
Kyrie, eleison!
Christe, eleison!
Kyrie, eleison!
Lord, have mercy on us.
Christ, have mercy on us.
Lord, have mercy on us.
3. Graduale: Requiem aeternam
Requiem aeternam dona eis, Domine:
et lux perpetua luceat eis.
In memoria aeterna erit iustus,
ab auditione mala non timebit.
Eternal rest grant unto them, O Lord :
and let perpetual light shine upon them.
He shall be justified in everlasting memory,
and shall not fear evil reports.
4. Tractus: Absolve, Domine
Absolve, Domine,
animas omnium fidelium defunctorum
ab omni vinculo delictorum
et gratia tua illis succurente
mereantur evadere iudicium ultionis,
et lucis aeternae beatitudine perfrui.
Forgive, O Lord,
the souls of all the faithful departed
from all the chains of their sins
and by the aid to them of your grace
may they deserve to avoid the judgment of revenge,
and enjoy the blessedness of everlasting light.
5. Sequentia: Dies Irae
Dies irae, dies illa
Solvet saeclum in favilla,
Teste David cum Sibylla.

Quantus tremor est futurus,
Quando judex est venturus,
Cuncta stricte discussurus!

Tuba mirum spargens sonum
Per sepulcra regionum,
Coget omnes ante thronum.

Mors stupebit et natura,
Cum resurget creatura,
Judicanti responsura.

Liber scriptus proferetur,
In quo totum continetur,
Unde mundus judicetur.

Judex ergo cum sedebit,
Quidquid latet apparebit.
Nil inultum remanebit.

Quid sum miser tunc dicturus?
Quem patronum rogaturus,
Cum vix justus sit securus?

Rex tremendae majestatus
qui salvandos salvas gratis
sale me, fons pietatis

Recordare, Jesu pie,
Quod sum causa tuae viae:
Ne me perdas illa die.

Quaerens me, sedisti, lassus;
Redemisti crucem passus;
Tantus labor non sit cassus.

Juste Judex ultionis,
Donum fac remissionis
Ante diem rationis.

Ingemisco tanquam reus,
Culpa rubet vultus meus;
Supplicanti parce, Deus.

Qui Mariam absolvisti,
Et latronem exaudisti,
Mihi quoque spem dedisti.

Preces meae non sunt dignae,
Sed tu, bonus, fac benigne,
Ne perenni cremer igne.

Inter oves locum praesta,
Et ab hoedis me sequestra,
Statuens in parte dextra.

Confutatis maledictis
Flammis acribus addictis,
Voca me cum benedictus.

Oro supplex et acclinis,
Cor contritum quasi cinis,
Gere curam mei finis.

Lacrimosa dies illa,
Qua resurget ex favilla
Judicandus homo reus.

Huic ergo parce, Deus:
Pie Jesu Domine:
Dona eis requiem. Amen.
This day, this day of wrath
shall consume the world in ashes,
as foretold by David and the Sibyl.

What trembling there will be
When the judge shall come
to weigh everything strictly!

The trumpet, scattering its awful sound
Across the graves of all lands
Summons all before the throne.

Death and nature shall be stunned
When mankind arises
To render account before the judge.

The written book shall be brought
In which all is contained
Whereby the world shall be judged

When the judge takes his seat
all that is hidden shall appear
Nothing will remain unavenged.

What shall I, a wretch, say then?
To which protector shall I appeal
When even the just man is barely safe?

King of awful majesty
You freely save those worthy of salvation
Save me, found of pity.

Remember, gentle Jesus
that I am the reason for your time on earth,
do not cast me out on that day

Seeking me, you sank down wearily,
you saved me by enduring the cross,
such travail must not be in vain.

Righteous judge of vengeance,
award the gift of forgiveness
before the day of reckoning.

I groan as one guilty,
my face blushes with guilt;
spare the suppliant, O God.

Thou who didsn't absolve Mary [Magdalen]
and hear the prayer of the thied
hast given me hope, too.

My prayers are not worthy,
but Thou, O good one, show mercy,
lest I burn in everlasting fire,

Give me a place among the sheep,
and separate me from the goats,
placing me on Thy right hand.

When the damned are confounded
and consigned to keen flames,
call me with the blessed.

I pray, suppliant and kneeling,
a heart as contrite as ashes;
take Thou my ending into Thy care.

That day is one of weeping,
on which shall rise again from the ashes
the guilty man, to be judged.

Therefore spare this one, O God,
merciful Lord Jesus:
Give them rest. Amen.
6. Offertorium: Domine, Jesu Christe
Domine, Jesu Christe, Rex gloriae,
libera animas omnium fidelium defunctorum
de poenis inferni
et de profundo lacu.
Libera eas de ore leonis
ne absorbeat eas tartarus,
ne cadant in obscurum;
Sed signifer sanctus Michael
repraesentet eas in lucem sanctam,
Quam olim Abrahae promisisti
et semini eius.

Hostias et preces tibi, Domine
laudis offerimus
tu suscipe pro animabus illis,
quarum hodie memoriam facimus.
Fac eas, Domine, de morte
transire ad vitam.
Quam olim Abrahae promisisti
et semine eius.
Lord Jesus Christ, king of glory,
deliver the soulds of all the faithful departed
from the pains of Hell
and the bottomless pit.
Deliver them from the jaws of the lion,
lest hell engulf them,
lest they be plunged into darkness;
but let the holy standard-bearer Michael
lead them into the holy light,
as once you promised to Abraham
and to his seed.

Lord, in praise we offer you
Sacrifices and prayers,
accept them on behalf of those
who we remember this day:
Lord, make them pass
from death to life,
as once you promised to Abraham
and to his seed.
7. Sanctus (+ Benedictus)
Sanctus, sanctus, sanctus
Dominus Deus Sabaoth!
Pleni sunt coeli et terra gloria tua.
Hosanna in excelsis!
Holy, holy, holy
Lord God of hosts!
Heaven and earth are full of your glory.
Hosanna in the highest!
Benedictus qui venit in nomine Domine.
Hosanna in excelsis!
Blessed is he that cometh in the name of the Lord.
Hosanna in the highest!
8. Agnus Dei
Agnus Dei, qui tollis pecatta mundi
dona eis requiem.
Agnus Dei, qui tollis peccata mundi,
dona eis requiem sempitername.
O Lamb of God, that takest away the sins of the world,
Grant them rest.
O Lamb of God, that takest away the sins of the world,
Grant them eternal rest.
9. Lux Aeterna
Lux aeterna luceat eis, Domine,
cum sanctis tuis in aeternum,
quia pius es.
Requiem aeternam dona eis Domine,
et lux perpetua luceat eis,
quia pius es.
Let everlasting light shine upon them, Lord,
with Thy saints for ever,
for Thou art merciful.
Grant them eternal rest, Lord,
and let perpetual light shine upon them,
for Thou art merciful.
10. Pie Jesu
Pie Jesu, Domine, dona eis requiem.
Pie Jesu, Domine, dona eis requiem, sempiternam.
Merciful Jesus, O Lord, grant them rest.
Merciful Jesus, O Lord, grant them eternal rest.
11. Libera me
Libera me, Domine, de morte aeterna
in die illa tremenda
quando coeli movendi sunt et terra,
dum veneris judicare saeculum per ignem.

Tremens factus sum ego et timeo,
dum discussion venerit atque venture ira:
quando coeli movendi sunt et terra.
Deliver me, O Lord, from eternal death
on that awful day
when the heavens and earth shall be shaken
and you shall come to judge the world by fire.

I am seized with fear and trembling
until the trial is at hand and the wrath to come:
when the heavens and earth shall be shaken.
12. In Paradisum
In paradisum deducant angeli;
in tuo adventu suscipiant te martyrus
et perducant te in civitatem sanctam Jerusalem.

Chorus angelorum te suscipat
et cum Lazaro, quondam paupere,
aeternam habeas requiem.
May the angels lead you into paradise;
at your coming may the martyrs receive you
and lead you to the holy city of Jerusalem.

May the chorus of angels receive you
and with Lazarus, once poor,
may you have eternal rest.