people's jobs: He's ___ architect. / She's ____ scientist. / My grandmother was ___ teacher.

there’s only one of something: This company is very profitable. ___ manager must be really good.  / ____ moon looks beautiful today.

we mention something for the first time: I saw ___ old woman with ___ dog in the park.

before plural or uncountable nouns to talk about something in general: I don’t drink ___ milk. / ___ children need a lot of sleep.

to say what kind of person or thing something/somebody is: Paris is ___ interesting city.

we mention something for the second time: A man and a woman sat in front of me. ___ man was British, but I think ___ woman wasn’t.

to talk about specific things or people / it’s clear which things or people we are talking about: ‘Where are ___ kids?’ ‘They’re in ___ garden.’ (=We know which kids and which garden.)

Can you open ___ door? (=We know which door.)

With places in a town where we commonly go: I’m going to ___ bank. / I found Peter at ___ station./ I’m at ___ library.

before superlative adjectives: This is ___ best restaurant in town.

before names of musical instruments when referring to it in general : Margaret plays ___ guitar and I play ___ flute. 

I love ___ music. (=Music in general.)

I loved ___ music at the party. (=Specific music.)

___ children are often difficult. (=Children in general.)

They are a nice family. ___ children are very funny. (=___ children in that family.)

before the names of meals (breakfast, lunch, or dinner): ___ dinner is served at 8.

I always have ___ breakfast with my children.

before the names of languages and school subjects: He doesn’t speak ___English.

I study ___ biology. 

before next and last + time expression (when they mean before or after now): The meeting is ___ next Thursday.

I saw him ___ last week.

__ last year we spent one week in London (___last year= the year before now).

We really loved London. ___ last day we were really sad. (___ last day= ___ last day of that trip. NOT the day before now)

something that is part of a group or type: Do you want ___ drink?

talk about a particular group of things: We went to the zoo and saw ___ kangaroos. (These are ___ particular kangaroos in that zoo – not kangaroos in general.)

because there is only one in that context:

We live in a small house next to ___ church. (= ___ church in our village) / Dad, can I borrow ___ car? (= ___ car that belongs to our family)

to say something about all the things referred to by a noun:

___ wolf is not really a dangerous animal. (= Wolves are not really dangerous animals.) / ___ kangaroo is found only in Australia. (= Kangaroos are found only in Australia.) / ___ heart pumps blood around the body. (= Hearts pump blood around bodies.)

to refer to a system or service:

How long does it take on ___ train? / I heard it on ___ radio. / You should tell ___ police.

with TV (when used as a broadcasting service, NOT as an appliance): I saw it on ___TV. / I don’t watch ___TV.

Turn off ___ TV.

I’ve bought ___ new TV.

with adjectives like rich, poor, elderly and unemployed to talk about groups of people: 

Life can be very hard for ____ poor. / I think ____ rich should pay more taxes. / She works for a group to help ___ elderly.