"How does he last?"
Translation:Comment est-ce qu'il dure ?
You are perfectly right:
"comment dure-t-il ?" belongs to the more formal register;
"comment est-ce qu'il dure ?" to standard register (but it is not recommended to use both an interrogative word like comment, pourquoi, où... and est-ce que);
"il dure comment ?" to the relaxed/oral register.
With verbs of the 1st group (infinitive in -er, except aller), the inversion Verb-Subject creates a hiatus in pronounciation with pronouns starting with a vowel (il/elle/on). To cope with it, we insert a T between hyphens, so that it better flows:
- mange-t-il ? dure-t-elle ? parle-t-on ?
This happens often, namely with verbs of the 1st group (infinitive ending in -er):
To ease pronounciation for the speaker and understanding for listeners, a T is added (between hyphens) when the verb ending is a vowel and the subject pronoun is placed after and starts with a vowel, i.e. all three "il, elle, on".
present tense: - quand danse-t-elle ? à qui parle-t-il ? que mange-t-on ? où va-t-il ?
future tense (all verbs): - chantera-t-elle ? ira-t-on ? viendra-t-il ? finira-t-elle ?
Btw, @saycats -- @Sitesurf answered just this question 1 month ago in this very discussion board (-:.
Though I like the idea of asking her the same questions repeatedly to see whether she gives consistent answers. If even she cannot, then we know French is truly hopeless and we can all just make it up as we go along. <-;