Will the -s at the end of "très" be always pronounced?

Question

I have been thinking that if a word beginning with a vowel (a, i, u, e, o) follows right after a word ending in a consonant then this consonant will be pronounced.

Example.

• Je suis une personne.
  Here, will the word "suis" be pronounced as "suis" without s ?
• Je suis très sociable. In this second example will "suis" be pronounced as "sui"?

But how about "très optimiste"? Will the -s in "très" be pronounced too?

Answer 1

A word ending with a consonant followed by a word starting with a vowel is necessary but not sufficient for a liaison.

This rules out any liaison in:

Je suis très sociable. Neither the final s of suis, nor the final s of très is pronounced.

That doesn't mean final consonants are never pronounced in French. There are many words where they are, but this is unrelated to your question.

A potential liaison belongs to one of three groups:

• Mandatory liaisons. Not doing them is considered a mistake whatever the language register.

Les avions (the planes). Always pronounced with a liaison, no exception.

Je suis très optimiste. Liaison may be missed in colloquial relaxed French, depending on the speaker.

• Optional liaisons. These are more often realized in formal than in colloquial speech.

This group is the larger one:

Je suis une personne. My impression is that a liaison is often not realized.

• Forbidden ones. Realizing such a liaison is considered a mistake.

Le président autrichien.

Note that what group a liaison belongs to is not set in stone. There have been changes in the past and there are a few ongoing ones.

For example Wikipedia states the liaison after très is optional but in other sources, e.g. BDL and FLE, it is mandatory.

Finally, note that semi-vowels are definitely compatible with liaisons, e.g.:

/j/ Dans tes yeux. (mandatory liaison)

/ɥ/ Une sauce sans huile. (mandatory liaison)

/w/ Il respire avec ses ouïes (mandatory liaison)

As usual, there are exceptions:

Les oui et les non (forbidden liaison¹)

---

Also, note that an h can influence the liaison, depending on whether its an aspirated h ("h aspiré") or not. Both are silent, but the aspirated h prevents the liaison. Normal rules apply if the h is not aspirated.

Des histoires (not aspirated, liaison)

Des hiboux (aspirated, no liaison)

You can find the complete list of words with an aspirated h on the wikipedia page.

¹ _{Oui behaves exactly as if it had started with an aspirated H: "Houi"}

Answer 2

It is pronounced, but only in those cases when a liaison is required and then its pronunciation is not the expected one (/s/) but /z/.

• Je suis allé dans ce magasin. /ʒə sɥizale dɑ̃…/

The liaison is required before vowel sounds but not before semi-vowels sounds (/j/, /ɥ/, /w/).
(« […] généralement, en français, les semi-voyelles ne donnent pas lieu à élision ou liaison. », réf.)

The semi-vowels in French and typical words in which they are found

/j/, comme dans yeux [jø], fille [fij], abeille [abɛj];  

/ɥ/ comme dans huile [ɥil], lui [lɥi] ;  

/w/ comme dans oui [wi], jouet [ʒwɛ], moi [mwa].

There are a great number of exceptions and some cases when it is not considered to be an error not to make a liaison although certain people think it is better to respect those liaisons. Here is a good introduction to the question of liaisons: liaisons.

Examples

• Je suis au téléphone, ne me dérange pas. /ʒə sɥizo te.le.fɔn…/

• Je suis oisif. /ʒə sɥi wa.zif/

• Je suis ailleurs. /ʒə sɥiza.jœʁ/

It follows then that in "je suis optimiste" the s is pronounced, just as in "très optimiste".

• /ʒə sɥizɔp.ti.mist/
• /\tʁɛzɔp.ti.mist/