Spelling in Italian is simple: once you know the basic rules you’ll be able to read a word and pronounce it with a fair amount of accuracy. Knowing where to put the accent, however, is not always so straightforward. Let’s have a look at the rules for using accents in Italian.

All words have accents, or use the accent of a neighbouring word. Some monosyllabic words, such as articles, use the accent of the word that follows:
il cane = the dog
la macchina = the car
… or the preceding word
lavati = wash yourself
ascoltami = listen to me

Lavati! Image CC0 Public Domain

When we pronounce words with accents our voice puts more emphasis on the accented syllable. This is known as a tonal syllable (from the Greek tònos = force). Syllables without accents are known as atonal.

Words are divided into the following categories according to the position of the syllable:

1. tronche (truncated) = with the accent on the final syllable:
virtù = virtue
realtà = reality
These are words that have been truncated, deriving from words that have had the final syllable chopped off, e.g. virtù comes from virtude.

2. piane (plain) = with the accent on the penultimate syllable:
giorno = day
frutta = fruit
amore = love
These words are called plain or smooth (liscia) because they are easy to pronounce, and they constitute the largest group of Italian words.

3. sdrucciole (slippery) = with the accent on the third syllable from the end:
popolo = the people
forbici =scissors
albero = tree
These are called ‘slippery’ because your tongue slides over the two syllables following the tonal or accent.

4. bisdrucciole = (lit. twice slippery) with the accent on the forth syllable from the end:
capitano = they happen, e.g. queste cose capitano (these things happen)
considerano = they consider
These are all verbal forms in the third person plural.

5. trisdrucciole = (lit. thrice slippery) with the accent on the fifth syllable from the end:
recitamelo = recite it to me
These are verbal forms followed by atonal monosyllables, in the above case me (me) and lo (it)

In written Italian, you must write the accent on parole tronche (truncated words), e.g. farà (he will do), sarò (I will be), and on monosyllabic words that need to be distinguished from their homophone. Here are the main ones:
è (is) as opposed to e (and)
sé (oneself) as opposed to se (if), although if sé is reinforced with stesso the accent can be omitted, e.g. l’ha fatto per sé (he did it for himself) or l’ha fatto per se stesso (he did it for himself)
né (neither/nor) as opposed to ne (of it/them)
sì (yes) as opposed to si (the impersonal or reflexive pronoun)
dì (day) as opposed to di (of/by etc.)
lì (there) as opposed to li (them)
là (there) as opposed to la (the, or her)
dà (give imperative) as opposed to da (from, by, to, etc.)
ché (abbreviation of perché) as opposed to che (that, which, whom, etc.)

Some words change meaning depending on the placement of the accent:
capitano (captain) vs capitano (they happen)
ancora (anchor) ancora (again)
In these cases we only write the accent if the meaning of the word isn’t clear from the context.

Here are some more articles about accents that you may like:
Parole e Accenti
It’s All In The Accent  
It’s All In The Accent – Part 2.