Articles:

- a/an:

* we use when the first time we mention a person o thing.

* professions 

*expressions of frequency

Examples: 

-Mary's an economist

- Once a week

- The:

*we use the next time because it is now clear who or what we are talking about

* when there is only one of something 

* before superlative adjectives

*when the speaker and hearer know the thing we are talking about

Examples: 

-open the door, please

-The sun came out so we went to a walk

-Roberto is the best student in him class

- No article:

*when we talk about people or things in general

 * before possesive's

*with meals, places and by + transport

Examples:

-She had breakfast this moorning

-Susana doesn't like cats

-Last year Ximena went on holiday by plane.