Definite article - use

The definite article is used:

with common nouns denoting

unique entities: il sole, la terra, la luna

something that is familiar to both speakers

generic reference: il vino fa male allo stomaco (wine is bad for your stomach)

with parts of the body if the part is a complete set or a unique entity: mi fa male la testa, ho 

i capelli biondi (but mi fa male un dente)

with clothes, physical conditions, possessions, rituals, family members, etc.

with geographical names: countries, regions, continents, large islands, seas, oceans, rivers, mountains

but no article with preposition in with feminine names of countries

L´Italia but in Italia

With masculine names – in / nel Portogallo

with preposition di – d´Italia – Italian

dell´Italia – of Italy

with plural names – always with article – Gli Stati Uniti – negli Stati Uniti

with names of languages: l´italiano, il giapponese

with the construction faccio + profession – Faccio l´insegnante – I am a teacher

The article is not used

with small islands: Capri, Ischia

with names of cities, towns, villages, (exceptions where the article is part of the name: Il Cairo, 

L´Aquila, La Spezia, L´Aia, La Valletta, Il Pireo, La Mecca, L´Avana)

in verbal expressions: aver sonno, far piacere, aver ragione / torto, far paura, prendere nota, prendere sonno, etc.

in exclamations: Povero ragazzo!

after certain prepositions such as: da, come, in quanto – da paziente, in qualità di preside

when defining the function or nature of something / somebody: 

Mio padre è insegnante. My father is a teacher.

in phrases: a scuola, in ufficio, a teatro, in tasca, in mano, a tavola, in banca, in paradiso, in comune, in macchina, etc.