Latin writers and speakers may “throw in" interjections to express strong emotion. Latin employs some of English’s interjections as well as some that do not appear in English. As with all new vocabulary, their meanings must simply be memorized. However, unlike most Latin words, interjections have only one form; they do not decline or change form since they can only be used one way in a sentence. For example:
Ah, nunc intelligo. (Ah, now I understand.)
Oh, virum nunc in via video. (Oh, now I see the man in the street.)
Heu, illa puella pulchra est. (Alas, that girl is beautiful.)
One mistake Latin students often make is mixing up the use of an interjection with the use of the vocative case. Both constructions use a short word such as “oh" at the beginning of a sentence, but they are used to express entirely different meanings.
The vocative case (from Latin “vocare", to call) is use to indicate a person or persons spoken to. Often, the word “oh" is used to differentiate the vocative case from other cases. This is because the vocative case has a similar form to other cases of the same declension. For example:
Oh Caesar, inimicum superavisti. (Oh Caesar, you defeated the enemy.)
Notice that “Caesar" is in the vocative case but has the same form as the nominative case. The “oh" is used to indicate that Caesar is being spoke to rather than being the subject of the sentence. Distinguishing the vocative case from the use of an interjection is usually a matter of identifying how many words appear before the comma. One word before a comma usually indicates an interjection while two words often indicate the vocative case. Keep in mind that the word “oh" is not always used before the vocative case, but is usually included in elementary Latin textbooks and programs of study to help with its identification.