In most instances where English uses the definite article, French does too. However, it has some special uses in French, which I shall explain below.
1) Must be used with all abstract nouns (nouns referring to ideas, states, concepts etc. as opposed to definite physical objects) as well as with nouns being used in a general sense to represent an activity.
ex. “La vie est belle”
“L'alcool n'est pas bon pour la santé.”
2) Must be used when referring to official titles except when directly addressing that person.
ex. “Le président Obama vient d'arriver”
BUT “Comment puis-je vous aider, président Obama?”
In very formal address, however, the definite article IS used and appears before the title.
ex. “Je vous presente, Madame la Marquise.”
3) It is also used with continents, countries, islands, regions, provinces, rivers and mountains
ex. “Les Pyrénées sont entre la France et l'Espagne.”
It is only used with cities when the name of the city has been modified by an adjective.
ex. “J'adore la Paris nocturne.”
4) It is used with weights and units of measurement alongside a specific price (as in per)
ex. “Les oranges coutent 1.20 euros le kilo.”
5) Must be used with languages and academic subjects UNLESS preceded by de or en
ex. “J'étudie le français à l'Université de Manchester.”
BUT
“Je parle avec mes amis francophones en français.”
6) It is used with seasons, days of the week, dates, etc. in the sense of in, every or on
ex. “Je vais en ville avec mes parents le samedi”
“Il fait très froid l'hiver”
I hope this guide proves useful when you are preparing for the written paper and struggling to work out whether you need the definite article or not.