The Introit of today's mass is taken from the Prophet Isaias, with a verse from psalm 79.
Populus Sion, ecce Dominus veniet ad salvandas gentes: et auditam faciet Dominus gloriam vocis suae in laetitia cordis vestri. Ps. Qui regis Israel intende: qui deducis velut ovem, Joseph.
People of Sion, behold the Lord will come to save the Gentiles: and the Lord will make the glory of his voice heard to the joy of your hearts. Ps. Give ear, O thou that rulest Israel: thou that leadest Joseph like a sheep.
(Isaias 30:30 and Psalm 79:2 from the Introit of Mass)
Populus - People.
Sion - of Sion. Should be genitive case, indicating possession, but since Sion is a word of non-Latin origin its ending does not change.
ecce - behold! look!
Dominus - the Lord.
veniet - he will come. Venio is I come. Here the verb is in the future tense.
ad - unto. The noun or pronoun following it will take the accusative tense.
salvandas - 'the saving of'. This is a gerundive - a verbal adjective - see grammatical note below! Salvo is I save; the ending -andas indicates a gerundive in the accusative case, agreeing with gentes which it qualifies.
gentes - the peoples. Gens is people. Here Gentes is plural and in the accusative case, following ad.
et - and
auditam - 'heard'. This is a 'perfect passive participle.' It has the sense of 'having been heard.'
faciet - he will make. Facio is I make. Faciet is future tense.
Dominus - the Lord. Subject of the verb faciet.
gloriam - the glory. Gloria is glory. Gloriam is accusative case, as direct object of the verb faciet.
vocis - of the voice. Vox his voice. Vocis is genitive case, indicating possession.
suae - his. The ending changes to go with vocis, which it qualifies.
in - in. Followed by the ablative case when it indicates state or location.
laetitia - the joy. Ablative case, to follow in.
cordis - of the heart. Cor is heart. Cordis is genitive case, indicating possession.
vestri - your (plural). The ending indicates genitive case, going with cordis, which it qualifies.
Qui - Who. Relative pronoun. This makes the subject of the verb 'you who' rule.
regis - You rule. Rego is I rule. Regis is you rule (singular).
Israel - Israel. Another non-Latin word whose ending doesn't change here. It should be accusative case, as direct object of the verb regis.
intende - Give ear. Imperative form of the verb.
qui - Who. A relative pronoun.
deducis - you lead. Deduco is I lead. Deducis is you lead (singular).
velut - like.
ovem - a sheep. Ovis is a sheep. Accusative tense, to go with Joseph, as direct object of the verb deducis.
Joseph - Joseph. Another non-Latin word that doesn't change its ending. Direct object of the verb deducis.
Grammatical Note - The Gerundive
'ad salvandas gentes' - In order to save the peoples.
Salvandas is a gerundive. A gerundive is a verbal adjective - so it is used to qualify nouns as if it were an adjective. It has a passive sense, and expresses something that must or ought to be done. Salvandas means literally something like 'which is to be saved'. So the phrase means literally ' to the peoples which is to be saved' - more colloquially, 'in order to save the people.'
This is quite a common construction. Another example is:
Ad pacem petendam - in order to seek peace.
Ad - towards. Followed by accusative.
Pacem - peace. Pax is peace. Pacem is its accusative, to follow ad.
Petendam - this is the gerundive of peto, I seek, and has the sense of fitting or ought to be sought. The ending -am indicates agreement with the noun pacem.
Gerundives can be recognized since the end with -andus, -endus, -iendus - although the precise ending -us might be changed to agree with whatever noun has been qualified. So it might be -endam (as in petendam) or -andas (as in salvandas).
Amandus - fitting or ought to be loved. (Feminine form amanda, as in the name).
Monendus - fitting our ought to be warned or advised.
Regendus - fitting or ought to be ruled.
Audiendus - fitting or ought to be heard.