I know that liaison often happens with article + noun (les amis), adjective + noun (beaux amis), and personal pronoun + verb (ils ont).
But how about the situation of verb + preposition? e.g.
Kaboul est en Afghanistan.
Bangkok est en Thaïlande.
When speaking the sentences above, would there be a liaison between est and en?
There are three sorts of liaisons in French: mandatory, forbidden and optional.1
The liaison after est is mandatory and always pronounced:
Other liaisons after est are optional but they are more or less followed according to the nature of the word that follows est.
Very frequent:
when est is followed by a past participle:
Il est arrivé /ilɛtaʁive/
when est is followed by an attribute:
C'est une vieille histoire /sɛtyn/
Just frequent, often omitted in relaxed conversation, in most other cases, including in front of prepositions:
We note that the French language has been (and probably still is) operating a shift from mandatory to optional liaisons. For example the 10th edition of Le Bon Usage states that the liaisons between c'est and the following preposition, and between est and the following past participle are mandatory, but when we listen to people in France in the 21st century (I would not know about other French speaking countries and it might be different) there's a great tendency to drop it in casual speech.
1
- A guide to liaisons in French
- Quelques règles de liaisons
Both liaisons are optional in your examples.
As regards verbs, liaisons are mandatory in the following case:
Entre le pronom personnel (ainsi que "on", "en" et "y") et son verbe, ainsi que l'inverse : nous avons, elles aiment, on ouvre, ont-ils, prends-en, allons-y.
As well as in "c'est̲-̲à̲-dire".
