That's an interesting question Katie,
In the conjugated examples of the lesson , the only one where there's no liaison between the 't' and the 'est' is the 'tu' form and I can't justify why.
You just wouldn't say "Ça t'est_égal"
There are some liaisons that are compulsory,
on_a , vous_avez, un_ examen
and some forbidden like after 'et'
or to sound a 'h aspiré':
des héros, des haricots, en haut.
Maybe a lesson on this would be useful...
Hope this helps!
