How can I tell the declension of a word?

These are the type of questions im getting

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Choose all that apply to the noun pueri.

1st declension

2nd declension

3rd declension

Nominative

Ablative

Singular

Plural

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Can someone help and explain it too me??

there is one guy in the helper list: http://forums.d2jsp.org/user.php?i=300371 whom you could pm

i dont even remember what an ablative is. wtf is this shiet.