SPECIAL USES OF "THE"

THE  + adjectives

We use THE with adjectives to talk about general groups of people having that quality (you don’t need a noun). In this case we use a plural verb.  

- The young = the young people
- The homeless are specially vulnerable in winter
- Do you think the rich should pay more taxes to help the poor?
 
More common examples:  the young / the old / the elderly / the poor / the unemployed / the homeless /  the sick / the disabled / the injured / the dead

We also use this same construction with some nationality adjectives with no equivalent noun:
 
- The British are worried about violence in schools
- There was a conflict between the English and the Welsh
 
Other nationalities have a noun, so they don't use that construction:
- Germans are interested in new markets (not: The German are...)
- Russians have a very complicated history
- Spaniards have one of the best public health systems in the world

  We use the possessive adjective instead of "the" when talking about parts of the body.
 
- Me va a estallar la cabeza = My head is going to explode
- Baja las piernas = Put your legs down
- Tienes la cara verde = Your face is green

SPECIAL USES OF "A"

We use A meaning "per" (distributive)

- That’s two pounds a kilo = that's two pounds per kilo (every kilo is two pounds, so 3 kilos is 6 pounds)

= Son dos libras el kilo (en español se usa en este caso el artículo determinado, no el indeterminado)

- He was driving at sixty miles an hour
- I go to the cinema twice a month (= two times every month)
 
Before singular count nouns in exclamations.

- What a day!  (countable singular)
- What a pretty girl!  (countable singular)
but:
- What nice weather!  (uncountable)
- What pretty girls!  (countable plural)

exception: What a pity! (uncountable)   [= ¡qué lástima!]
 
We often suppress the article (or possessive) after the conjunction "and" in a pair of things that often go together.

- I need a knife and fork
- He went out with a raincoat and umbrella
- Those are the king and queen
- My uncle and aunt came to visit me
but:
- I found a hat and a camera in the car  (not: a hat and camera)