The prefix 's-' that works like English 'un-' and 'dis-'
Italian has a small but very useful trick: put an s in front of certain words and the meaning flips. Fortunato (lucky) becomes sfortunato (unlucky), vantaggio (advantage) becomes svantaggio (disadvantage), and comparire (to appear) becomes scomparire (to disappear). With verbs, the same s- often means undoing an action, as in slegare (to untie) or sbottonare (to unbutton).
English speakers tend to miss these words for two reasons. The prefix is only one letter, so the link to the positive word is easy to overlook. Also, English spreads the same idea across several prefixes (un-, dis-, im-), while Italian uses s- for all of them in these pairs. Once you notice it, many new words become easy to guess.
The prefix attaches directly to a word that starts with a consonant: s + cortese gives scortese, s + gradevole gives sgradevole. There is no hyphen and no extra vowel. The result usually matches one of these English patterns:
The opposite quality, like English un- or dis-: fortunato becomes sfortunato, vantaggio becomes svantaggio.
Undoing what the base verb does, like English un-: vestirsi (to get dressed) becomes svestirsi (to get undressed).
Rule of thumb: if you meet an unfamiliar word starting with s plus a consonant, try removing the s. If what is left is a word you know, the new word probably means its opposite. Other classic examples are scomodo (uncomfortable) and sleale (disloyal).
Sfortunato describes a person or situation, and sfortuna is the noun, bad luck. Svantaggio is the everyday word for a disadvantage.
Sconosciuto is the opposite of conosciuto (known). As an adjective it means unknown; used as a noun, uno sconosciuto is a stranger.
Sgradevole is the opposite of gradevole (pleasant), and scortese is the opposite of cortese (polite). You will also hear spiacevole, from piacevole, with the same meaning as sgradevole.
These verbs reverse a physical action: slegare (to untie), sbottonare (to unbutton), svestirsi (to get undressed) and scartare (to unwrap, literally to take the paper, carta, off). Scomparire reverses comparire, and scoraggiare takes away coraggio (courage).
S- only works in established pairs. The opposite of felice is infelice, not a made-up form with s-, and the opposite of onesto is disonesto. Use the prefix to understand words, and check a dictionary before creating one.
In scrivere, stare or scegliere, the s is simply part of the word. The guessing trick only works when removing the s leaves a real word with a related meaning.
Scartare also means to discard or reject, for example an idea or a card in a game. Scomparire takes essere in compound tenses: sono scomparse, not a form with avere.
When an Italian word starts with s plus a consonant and you recognise the rest, it often means the opposite: fortunato becomes sfortunato, vantaggio becomes svantaggio, and legare becomes slegare. Think of it as English un- or dis-. Use it to recognise words, not to create new ones, because only established pairs exist.