Informally, il est sounds like yé. So il est jeune will sound like yé jeune when said spontaneously in a conversation.
But what about when the next word after est begins with a vowel, like in il est en train de…? In this case, you’ll hear a t sound, which comes from the final t of est.
Il est en train de will sound like yé t’en train de.
Remember, the liaison changes the pronunciation of the following word, not the word in which the normally silent letter appears.
So don’t say yét / en / train / de.
Say yé / t’en / train / de.
If you pause where you see “/”, you’ll hear the difference.
The same goes for vous avez, for example.
Don’t say vouz / avez.
Say vou / z’avez.
For les autos, don’t say léz / auto.
Say lé / z’auto.
