Some languages use copulas differently, so a sentence like "Bob old" would be a complete and grammatical expression (e.g. in Chinese "bobu lao" ըB, where "lao3" is a so-called "status verb" meaning "being old"), while they might still use a copula to associate one noun with another as in the "fireman" example (in modern Chinese, here the copula "shi" O is necessary). There are still many other languages e.g. Russian or Hungarian? where nouns don't have a copula between them (Russian: "ja chelovek" - I'm a man/human -, Hungarian: " ember" - he is a man/human). The artificial language Lojban has no copula at all, because all words that express a predicate can be used as verbs. The three sentences above would all have the same form in Lojban: "la bob. bajra", "la bob. tolcitno", and "la bob. fagdirpre". Some languages also have more than one copula for different uses; for example the Spanish language uses the verb "ser" to link a subject to a predicate that specifies an identity or inherent quality ("Bob is old" would be "Bob es viejo"), and the verb "estar" to link predicates that specify a temporary condition ("Bob is here" would be "Bob est aqu"). The Italian language does it the same way ("Roberto vecchio"/"Roberto sta qui"), and Hungarian? only uses a copula in the latter case with regard to 3rd person (sg/pl) (Rbert van itt"), but not in the 1st example ("Rbert reg"). This is to relate a subject to a more temporary condition/state taking place in *space* (very often in the sense of Lojban "zvati").
It should be pointed out that the English verb be can also be used as a standalone predicate when it is used to express mere existence. "I am" is a complete sentence meaning "I exist", and in this case "am" is not a copula but a normal, meaningful verb.
/Talk?