This type of mistake occurs so frequently that it should really be placed at number one.
The plural of words ending in E is always with an I, except for foreign words or words ending with an accent, for instance:
Il caffè (singolare)
I caffè (plurale)
To form the plural of a word ending in E, you need to replace the E with an I, for example:
È un mio parente
He is a relative of mine
Sono miei parenti
They are relatives of mine
The majority of students forget this rule, especially when it comes to agreements:
Mela verde (singolare)
Mele verdi (plurale)
Casa grande (singolare)
Case grandi (plurale)
Allora ricordate, il plurale dei sostantivi (nomi) e degli aggettivi che terminano in E si forma con la I!