The English, Spanish and French 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

Exceptions:

*note:
Remember that there isn't a liason with the word 'et'

1: un changes to une before a feminine, singular thing (noun) and with other numbers that use it, 21, 31 etc.

5: You don't pronounce the final consonant before another consonant (except in dates)

6: Pronounce it like the English word "cease" when it's alone, otherwise pronounce it with / without a liason as normal, before other words

8: You don't pronounce the final consonant before another consonant (except in dates)
Also don't use l', e.g. le huit de... (same with le huitième)

9: You don't (usually) pronounce the final consonant before another consonant (except in dates).
Also you sound the 'f' as a 'v' before 'ans' and 'heures', and as an 'f' before another word that sounds like it starts with a vowel (whether it does or not)

10: Pronounce it like "diiss" when it's alone, otherwise pronounce it with / without a liason as normal, before other words

11: Don't use l', e.g. le onze avril (same with le onzième)

20-29:
It is difficult to say whether the t of vingt is pronounced as different regions of France pronounce it when others don't.
All I can recommend is not to pronounce it except in a liaison:
20: vingt (pronounced a bit like a nasal'vah')
21: vingt et un (pronounced a bit like vah tay un)
22 - 29: no t because it is difficult to sound the t when the following word starts with a consonant
vingt euros (pronounced a bit like vah turow)

There is no liaison in dates e.g. le vingt avril (it sounds like a nasal 'vah')

80: Notice that this has an s (quatre-vingts)

81: No 'et' with this (quatre-vingt-un)

100, 200 etc:
When cent is multiplied by a number and when it is not followed by another number, it takes an s.
deux cents (multiplied by another number)
deux cent quatre (followed by another number)

You don't use 'un' or 'et' with this either e.g. deux cents, deux cent neuf, cent douze.
Liaisons:
cent euros (push the t in to euros)
deux cents euros (push the s as a z sound in to euros)

1000: No plural, and you don't use 'un' or 'et' with it either (mille euros, mille un). Be careful with 'le mille' which is a noun meaning 'mile' which does have a plural. Also watch out for:
mille deux = 1002
deux mille = 2000

1000000: Use 'de' directly before a noun (1000000 + de + noun) and it has a plural
un million de pixels
un million cinq cent mille pixels
deux millions d'euros = 2 million euros

Remember that France's usage of the comma instead of the decimal point in English, and the usage of spaces which separate three digits at a time. For example, one point five is 1,5 and a million is 1 000 000 (sometimes written with full stops, formally)