4.6 Words ending on co / go and word stress on the before last syllable
A glance on the endings, provided that you remember what we said in chapter 2.5 The 'c' (ca / co / cu is pronounced ka / ko / ku, ci / ce became tschi / tsche), is enough to realise that there is a problem. If you form the plural following the general rule the pronunciation changes. Tedesco (German) for instance would become (~Tedeci). Therefore it is necesary to put an h in between i order to avoid that the pronunciation of the c changes.
co => chi
go => ghi
But as you can see in the examples above there are words as well where the ca becomes ci (amico <=> amici).
If the word stress falls on the before last syllable, what happens very often in Italian (see 2.26) the c is maintained in his original phonetic value.
il tedesco => i tedeschi
the German => the Germans
l' albergo => gli alberghi
the hotel => the hotels
il parco => i parchi
the park => the parks
But there are some very important words that doesn't follow this rule.
l' amico => gli amici
the friend => the friends
l'amico forms the plural differently from l' amica