I have some troubles in understanding the syntax of a sentence from Plautus's Captivi, line 580; I need to add glosses. The sentence is "Nam is est servos ipse, neque praeter se umquam ei servos fuit". The translations for that sentence in different languages I have found are something like "because he is a slave himself, and there was never any slave in his possession except himself". However, I don't understand why "servos" is in accusative plural and not in nominative singular, as "is" is in the first part of the sentence. As far as I understand, the verb "sum" does not accept an accusative complement, even in it possessive meaning. The plural number is also for me really mysterious. Are these two uses of "sum" with "servos" examples in which "sum" takes the accusative? Am I confusing the case of "servos"? In the second part of the sentence, is "servos" the subject, or is it an impersonal sentence?
To expand a little on Joonas's answer, the nominative singular ending in Latin was originally /os/ for all masculine nouns of the second declension, which developed to /us/ as part of a more general sound change of /o/ to /u/ in certain positions. (Somewhat confusingly, Latin /u/ in turn corresponds to /o/ in a number of Romance languages. It's thought that Latin /u/ was phonetically realized as [ʊ], at least in certain accents and at certain time periods, while /o/ in Latin is thought to have had an open-mid quality like [ɔ] rather than a close-mid quality like [o]—I don't know to what extent these phonetic details are considered relevant to the interpretation of the /o/ > /u/ sound change.)
For a while, both "us" and "os" were used in writing, with the spelling "os" generally being conditioned by a preceding /w/ sound (written V; in modern texts usually written "v" or as "u" in "qu" or "gu"). The same variation applied to the ending "-om"/"-um". I don't know whether "o" represented the sound /o/ or /u/ in texts where it occurs as a conditioned variant of "u".
The "Orthography" section of Allen and Greenough's Latin Grammar says
The spelling of the first century A.D. (known chiefly from inscriptions) is tolerably uniform, and is commonly used in modern editions of the classics.
a. After v (consonantal u), o was anciently used instead of u (voltus, servos). This spelling was not entirely given up until the middle of the first century A.D.
b. The older quo became cu in the Augustan period; in the second century A.D. the spelling quu- established itself in some words:
cum (older quom1)
equos, ecus (later equus)
sequontur, secuntur (later sequuntur)
exstinguont, exstingunt (later exstinguunt)
Note— In most modern editions the spelling quu- is adopted, except in cum.
- The spelling quum is very late and without authority.
Plautus was an Old Latin writer (Wikipedia says "c. 254 – 184 BC"), and I haven't yet found a clear description of whether his use of -os is thought to have been mostly unconditional or mostly conditioned by /w/. Wikipedia says one of the "archaic features" of Plautus's plays is "the retention of short -ǒ in noun endings in the second declension for later -ŭ", not mentioning any kind of conditioning factors. But the Introduction to Plautus' Poenulus: A Student Commentary, by Erin Moodie, says "The nominative singular ending for nouns in the second declension is -os (with a short o) not -us, after consonantal u" (p. 27). I'm not sure how well extant manuscripts correspond to Old Latin pronunciation; according to An Introduction to Latin Textual Emendation, by W. M. Lindsay (1896),
The modernising of archaic forms is carried out to a great extent in the MSS. of Plautus, and the student of Plautine textual criticism must bestow great attention on this point.
-os, -om for -us, -um.—In Late Latin, as we have seen, u and o were in certain cases interchanged in pronunciation and writing; e.g. vinum was pronounced and written vino (cf. Italian vino). A Carolingian scribe might easily mistake a genuine form like equom or servos, nominative singular, for a Merovingian misspelling, and substitute equum, servus. In classical Latin o, the older vowel, was used after another u or after the consonant v, but in the Latin of Plautus' time the old spelling was retained in many other situations beside. It is not always easy to feel certain that an o which takes the place of u in the MSS. of Plautus is a genuine survival of the Old Latin form (e.g. opos sit for opus sit, which has become corrupted to possit in Stich. 573; see above, p. 3), or a mere instance of the Late Latin use of o for ŭ.
There was an earlier question about the words "secundus" and "equus" that touched on some of this: Why sequundus > secundus?
It is servŏs in both instances, not servōs. The old form of the nominative has the ending -os instead of the later -us. What you see is indeed the singular nominative, but not in the form you are used to.