Why is t’amusés incorrect

Question

The question is:  Invert ‘Tu t’es amusés.’ to be a question   = T’es tu amusés?  Why is the ‘u’ not dropped in “tu” as it is followed by a verb that starts with a vowel?

Maarten Kamp · Accepted Answer

T.S. 
Formal obligatory elision occurs only with a limited number of words ending in a vowel ( before a following vowel or h muet, and where relevant, the adverbial pronoun ‘ y ‘ ). 
These are mostly words ending in ‘e’, with exceptions being elision after the definite article and direct object  ‘la’, and with ‘ si ‘ when followed by ‘ il/s ‘ to give ‘ s’il/s ‘. 
In the example you note, the elision ‘t’es’ is derived from ‘ te es ‘, and is obligatory. An elision after ‘ tu ‘ would be informal in speech, and ‘ not correct ‘ in the written form. 
Formality would be expected here, as the use of an inverted question generally occurs in formal speech. 
Unless otherwise noted, the standard on this site is almost always to adhere to the formal rules
 https://progress.lawlessfrench.com/revision/glossary/contraction/l-elision-elision 
 
https://www.lawlessfrench.com/pronunciation/elision/
 
 https://www.lawlessfrench.com/grammar/informal-pronouns/

Cécile Janty-Davies · Answer

Bonjour T.S.
Just to add to Maarten's excellent answer, although there are some rare cases when in informal speech you might elide the 'tu' ( subject pronoun), you won't in the inversion.
For example -
T'es fou ou quoi ? = Tu es fou ou quoi ? = Are you mad or what?
In inverted form, it would be -
Es-tu fou ou quoi ?