Masculine - populus magnus
Nom. - populus magnus (the great people)
Gen. - populi magni (of the great people)
Dat. - populo magno (to/for the great people)
Acc. - populum magnum (the great people)
Abl. - populo magno (with/by/from the great people)
Voc. - popule magne (O great people!)
Nom. - populi magni (the great peoples)
Gen. - populorum magnorum (of the great peoples)
Dat. - populis magnis (to/for the great peoples)
Acc. - populos magnos (the great peoples)
Abl. - populis magnis (with/by/from the great peoples)
Voc. - populi magni (O great peoples!)
Sadly, masculine nouns sometimes do weird things. Like end in -er. The word for boy, puer, is a good example. Some adjectives, such as pulcher ("pretty") also act this way.
In the case of puer, you simply add the endings right on. Puer, pueri, puero, etc. The major exception to the 2nd declension paradigm are these -er nouns. In the vocative singular, they do not have an ending, but keep the nominative form. "PUER"
The adjectives that act like this most commonly drop the "e" before adding the stems. So: pulcher, pulchri, pulchro, etc.
When you see adjectives, they usually list the different gendered forms. Most, if not all, adjectives have forms for all three genders, and they are listed MASCULINE, FEMININE, NEUTER. The adjective magnus, listed with the two genders we now see, will appear as magnus, magna. When you see one of those nasty first declension masculine nouns, such as "poets", remember that it is in fact masculine, and that you need to use a masculine adjective form.
And now here is the bit about commas. Here's a little grammatical device known as apposition.
"The poet, Catullus, is giving roses to the girls."
See how poet and Catullus are both nominative? It is the same subject, but in two different nouns. In English, we separate them with commas. And we shall do so in Latin as well. They always agree in case, usually in number, and often in gender as well.
Ratey posed a good question in doing her last exercises (and thank you fot those, too!). If a noun has an I before the end, as "sententia", what do you do for the dative and ablative plural? Just keep the vowel, ad add the ending. Thus, "sententiis." (I told you wrong before, Ratey. Just clearing that up.)
Happy Latin!! Please check the vocab and grammar excercises for this chapter (chapter 3).