Latin of the Introits - Advent 4 - Rorate Coeli!

The first half of today's introit is from the book of  Isaias chapter 45, and recurs often in the liturgy at this stage of Advent. The second half is from Psalm 18.

Rorate coeli desuper, et nubes pluant Justum: aperiatur terra, et germinet Salvatorem. * Coeli enarrant gloriam Dei: et opera manuum ejus annuntiat firmamentum.
Drop down Dew, ye heavens, from above, and let the clouds rain the Just One: let the earth be opened and bud forth a Saviour. * The heavens show forth the glory of God: and the firmament declareth the works of His hands.

Rorate - Drop down dew. The verb is roro/rorate/roravi/roratum (remember the four main parts of a verb - they enable us to work out the endings). Here we have the imperative mood, since a command or request is being made. The -ate ending indicates plural imperative - a command is being made to the heavens.
coeli - O Heavens! This is the vocative case, since the heavens are being addressed. Heaven is coelum. In classical Latin it is only found in the singular, but in later and Church Latin the plural coeli is often used for heavens, plural.
desuper - from above.

et - and
nubes - the clouds. Cloud is nubes. The plural, used here, is also nubes. This is the subject of the verb pluant.
pluant - May they rain down. Pluo/pluere/pluit is the verb to rain. The ending indicates a subjunctive verb here, since it indicates a wish. (The most common use of the verb is simply pluit - it is raining.)
Justum - The Just one. Justus is the adjective meaning just, but can also mean a just man, or the just man. The -um ending indicates accusative tense, since this is the direct object of the verb pluant.

aperiatur - May it be opened. Aperio/aperire/aperui/apertum is the verb to open. Here we have a passive subjunctive form - passive since something is happening to the subject terra, and subjunctive since it is expressing a wish.
terra - The earth. Subject of the verb aperiatur.

et - and
germinet - Another subjunctive, expressing a wish. The verb is germino/germinare/germinavi/germinatum - to germinate, to sprint up. The subject of this verb is terra, the earth.
Salvatorem - A saviour. Salvator is saviour; the -em ending indicates an accusative, as the direct object of the verb germinet.

Coeli - The heavens. Subject of the verb enarrant.
enarrant - They tell forth. The verb is enarro/enarrare/enarravi/enarratum - to tell forth. The -ant ending indicates plural, to go with coeli.
gloriam - the glory. Gloria is glory. The -am ending indicates accusative case, as direct object of the verb enarrant.
Dei - of God. Deus is God. Dei is the genitive case, indicating possession.

et - and
opera - The works. Opus is a work. Opera is plural. This is the direct of the verb annuntio.
manuum - of the hands. Manus is hand. Manuum is plural, genitive case, indicating possession.
ejus - his.
annuntiat - It declares. The verb is annuntio/annuntiare/annuntiavi/annuntiatum - I declare.
firmamentum - The firmament. This is the subject of the verb annuntiat. The word order is a bit odd - you wouldn't usually expect the subject of a verb to come at the end of a sentence. When it does, it usually gives an emphasis to the word that is in the unusual position. Otherwise, it can be done for poetic effect, because it sounds good;  that, I think, is why it is done here.

Grammatical Note - Verbs of the First Declension.

Regular Latin verbs - those that follow a rule - follow one of four standard patterns in their endings. These are called the four declensions. The first declension is verbs with an -a- in their standard endings.

For example:
enarro/enarrare/enarravi/enarratum - I tell forth
roro/rorate/roravi/roratum - I drop down dew
germino/germinare/germinavi/germinatum - I spring forth/germinate
annuntio/annuntiare/annuntiavi/annuntiatum - I announce
amo/amare/amavi/amatum - I love.

Notice that all of these have -are/-avi/-atum in their standard endings in the dictionary. These all have the vowel -a- featuring prominently. They are said to have -a- stems.

Using amo as an example of how the endings work.

Present tense

Amo - I love
Amas - You love (singular)
Amat - He/she/it loves
Amamus - We love
Amatis - You love (plural)
Amant - They love

(Generations of schoolchildren have learned to chant this pattern off by heart: Amo, amas, amat, amamus, amatis, amant. Why not join a great tradition, and join them by learning it?)

Present tense, subjunctive

Amem - I may love

Ames - You may love (singular)

Amet - He/she/it may love

Amemus - We may love

Ametis - You may love (plural)

Ament - They may love

Future tense

Amabo - I will love

Amabis - You will love (singular)

Amabit - He/she/it will love

Amabimus - We will love

Amabitis - You will love (plural)

Amabunt - They will love

Perfect tense (simple past)

Amavi - I loved

Amavisti- You loved (singular)

Amavit - He/she/it loved

Amavimus - We loved

Amavistis - You loved (plural)

Amaverunt - They loved

I could go on, but that should give you an idea of the sorts of patterns that these verbs follow. If you want a full and proper grasp of Latin, at some point you will need to get hold of a Latin textbook or grammar book and master the ends of the verbs. Since Latin is an inflected language, it is largely in the changing endings that the meaning of the sentence is made clear. It's all very logical, but takes some time for a beginner to get his head round it all.