Inserting -t- after verbs ending in a t sound

Question

If you use the inverted form, the answer key asks for the extra -t- after s'adapte, so s'adapte-t-il. That looks redundant, since adapte already ends in a t sound. (https://french.kwiziq.com/revision/grammar/forming-questions-by-inverting-in-the-present-tense-il-elle-on-forms)
 
Is there perhaps a different rule for silent 'e' endings?

Accepted Answer

Adrian, 
The inclusion of the ‘ -t- ‘ in inverted questions between a verb form ending in a vowel, and a subject pronoun commencing with a vowel ( il, elle, on ) always applies. 
In formal spoken French, it has an impact on pronunciation, regardless of the usual pronunciation of the verb form. This includes a change in pronunciation with the verb forms ending  ‘ -te ‘, as noted in the link from Laura Lawless. For this reason, it can always be considered to be there for  ‘ euphonic ‘ reasons. 
 “ This includes when the verb ends in –te, which means that a schwa must be pronounced between the two t sounds:
    IPA  il monte>monte-t-il [mɔ̃ tə til]elle hante>hante-t-elle [ã tə tɛl]on écoute>écoute-t-on [e ku tə tɔ̃]Admittedly, on the recording here the ‘ schwa ‘ is quite subtle. It is consistent I suppose,  with the informal eliding of words/swallowing of vowels that occurs in everyday spoken French ! Regardless, in writing the correct form will be presented, especially as inverted sentences are of a more formal nature.
 
https://www.lawlessfrench.com/grammar/euphonic-inversion/ 
You might also be interested to read ( and listen to ) more about regional variations, and prononciation in poetry and songs
 https://medium.com/language-lab/merde-uh-the-sound-of-silent-e-a58f12baac87