Elision - Liaison

Reminder An open syllable ends in a vowel: A close syllable ends in a consonant: A word has as many syllables as it has vowels. The end of the pronounced syllable can not be the end of the written one.

The sequence

The ending and usually pronounced consonant of a word becomes the first consonant of the following word,
if this one begins in a vowel.
Il attend, elle arrive

The liaison

The ending and usually not pronounced consonant of a word becomes the first consonant of the following word,
if this one begins in a vowel.
Ils attendent neuf amis Compare with: ils prétendent

The Elision

The elision takes place when a word finishing in a vowel is followed by a word beginning in a vowel.
Grammatical words that end in -e lose it before a vowel. In writing, this -e is replaced by an apostrophe:
Je viens d'un (de + un) pays lointain.
J'attends (= je + attends) d'avoir l'occasion

The vowel -a can be dropped when it is in a personal pronoun before a verb beginning in a vowel:
Pierre aime Marie becomes Pierre l'aime (la + aime).

For the lexical words, -e disappears phonetically and only the vowel that begins the following word is pronounced.
The spelling does not change:
Quatre amis

Avoiding hiatus

As French language does not like that a vowel follows another vowel, some changes must be done sometimes. There are two solutions: either to drop a vowel (see elision) or to add a consonant to use it to make a liaison.

We add a consonant, usually t, in verbs. For example, we can inverse verb and pronoun to ask a question:
Nous appelons Pierre becomes Appelons-nous Pierre?
When the inversion makes two vowels following each other, we add a consonant to avoid the hiatus:
Il achète une maison becomes Achète-t-il une maison?

Usually, the rules do not apply when it is necessary to distinguish specific elements within a sentence.
un et un is pronounced: and never:

The liaisons take place inside groups of words: verbal phrases or noun phrases.
Listen to:
The sentence pronounced with stressed liaisons
The sentence pronounced naturally
[Le petit âne] [qui dort sous un pommier] [rêve] [qu'il devient un cheval].

(How natural grammar examples sound!)

When the noun phrase is a linked pronoun, the liaison must be done.
See above: Ils attendent