Liaison occurs when a mute final consonant (s, z, n, t) is connected with the vowel of the following word. If you think about this, you will notice that it only occurs when the preceding word ends in s, z, n or t, and the following word starts with a vowel or a mute h. If the mute final consonant happens to be an s, it is then pronounced in a voiced way, like in crazy.
Examples
French
correct and wrong
English
un
homme
a man
tout
entier
complete(ly)
avant
hier
day before yesterday
plus
ou moins
more or less
Comment
allez vous?
How are you?
bien
étrange
quite strange
Following some examples for the voiced s between two vowels:
Examples
French
correct and wrong
English
Vous
avez dormi.
You have slept.
chez
elle
at her place
nous
en avons
we have of it
très
utile
very useful
Now we come back to the problem of the unpronounced aspirated h:
A not-aspirated h has the liason, the merger between the consonant and the following vowel. An aspirated h does not have this liason. The words are not connected, but rather remain separated.
Listen to the examples!
les hommes
aber
les haches
For comparison both together:
Attention!
les
hommes <=>
les haches
If you now ask, how to know the difference, the answer is again in the dictionary. An aspirated h is marked with an apostrophe (').