The English and Spanish number system

Exceptions:

*note:
Some numbers require an accent mark to force the stress in to the same place as you would expect it to be (compare tres with veintitrés)

1: This has a number of forms which agree with the noun whether it's stated or implied:
uno (as a number)
un + masculine noun
una + feminine noun
So you may hear a sentence ending in "... y una" if it is implying a feminine noun.
There are different ways of expressing the concept of "some" (see this article). For a small number of countable things:
unos + a masculine plural noun
unas + a feminine plural noun
Han venido unos españoles = Some (or a couple) of Spanish people have come
¿Quieres unos libros? = Do you want some books?
Unos me gustan y otros no = Some I like, others I don't like
Los unos dicen que sí = Some say yes
Llegaron unos y se ducharon = Some people arrived and showered


21, 31 - 91 Numbers ending in uno
ochenta y una casas
Also change the spelling of ventiún + masculine noun. The accent is needed to force the stress in to the same place as the other variations.
There is some uncertainty when the numbers 21, 31, ... 91 are in front of another word. In writing, you always have an s on the noun e.g. veintiún libros but in speech you may hear it without the s.


100
Always use cien before a plural noun or before mil and before millón but in speech, cien and ciento are heard when spoken alone or in counting a series of numbers.
Ciento is used in percentages (el veinte por ciento) and before another number (ciento dos) but use cien at all other times (cien mil cinco).
100 = cien
101 = ciento uno
110 = ciento diez
1100 = mil cien (because cien is not before another number)
1101 = mil ciento uno
100 000 = cien mil
ciento dieciocho = one hundred and eighteen
mil cien metros = eleven hundred metres
cien millón = one hundred million

cien	ciento
100 on its own	before another number (ciento dos etc.)
before another noun	por ciento (but see the next section below)
cien mil (100 000)
cien millón

According to el Diccionario panhispánico de dudas, when ciento acts as a noun, cien is preferred. However, ciento is used in certain fixed constructions (phrases, proverbs etc.). The plural of ciento is cientos (don't use a plural of cien) e.g.
te lo he explicado cientos de veces = I've explained it to you hundreds of times.




200 - 900 have feminine forms e.g. trescientas casas, doscientas cincuenta mesas.

1000
No change before numbers e.g. ocho mil euros = 8000 euros but if you use it as a noun:
los miles de corredores que llegan a la vez = the thousands of runners that arrive at the same time

1000000
this uses de before nouns
un millón de insectos = million insects

notice that with some verbs, there's a stressed e→ie and a stressed o→ue, there's something similar going on with the numbers 70, 700 and 900

Remember that Spain's usage of . and , are the other way around so one point five is 1,5 and a million is 1.000.000