The critical question here is How does your teacher pronounce intervocalic
/r/?
In some quite prestigious dialects—including the Stage British
I was taught—/r/
falling between two vowels is pronounced as a voiced tip-flap.
That is, the tongue does not curve backward toward the roof of the mouth but touches the back of the upper teeth, or the alveolar ridge immediately above that point, once, very briefly.
The sound which this produces is virtually indistinguishable from the alveolar flap which most Americans use for intervocalic /t/, as in beautiful .
Indeed, I am not at all confident that these are two different sounds.
The alveolar flap is notated in IPA with the 'fish-hook r', /ɾ/ . Wikipedia notes—I have no idea whether this is directly relevant to your situation—that This sound is often analyzed (and therefore transcribed) by native English speakers as an 'R-sound' in many foreign languages.
For example, the 'Japanese R' in hara, akira, tora, etc. is actually an alveolar tap ∗ .
In languages where this segment is present but not phonemic, it is often an allophone of either an alveolar stop ([t] or [d]) or a rhotic consonant like the alveolar trill or alveolar
approximant.
∗ Note Wikipedia's assertion that “The terms tap and flap may be used interchangeably”.
ell.stackexchange.com/users/32/StoneyB on hiatus
What your (NS) teacher reportedly said does not reflect standard English pronunciation - the one that is usually taught in an L2 setting.
However, Wells 1984 reports a similar phenomenon in the same intervocalic position but between words in
northern (UK) dialects:
ell.stackexchange.com/users/1096/Alex B.
Beautiful is traditionally pronounced /bjuːtɪfʊl/ ( link ), but native English speakers often replace
/t/ sounds with /d/
sounds, so it often sounds like /bju:dɪfʊl/ .
With further elision due to omission of the glottal stop in some accents, this can sound like /bju:ɪfʊl/
or even /bjur:ɪfʊl/ - especially in the southern states of America and some North Eastern parts of England and Cockney English in London, which is possibly what you're hearing in this case.
That said, in my experience, most English speakers will pronounce beautiful as /bju:dɪfʊl/: https://www.youtube.com/watch?v=oofSnsGkops#t=048 https://www.youtube.com/watch?v=oe1wtkkt9-E#t=54 https://www.youtube.com/watch?v=eAfyFTzZDMM#t=053s
ell.stackexchange.com/users/307/Matt
Your teacher is mistaken.
It is pronounced with the "T" sound rather than the "R" sound.
ell.stackexchange.com/users/3190/Zachary
It is also possible that the pronunciation is where the 't' is a merely a glottal stop (common in British English, especially in the London area) rather than having been transformed into a 'd' sound (which is more likely the American version).
ell.stackexchange.com/users/2542/toandfro
Here is another video of a large number of AmE native speakers using the word beautiful.
Give it a try!
From the mouth of babes... https://www.youtube.com/watch?v=AHWPYJuDuZQ
ell.stackexchange.com/users/11216/Wichita Steve
It depends what language you're learning from as well.
The way Americans pronounce the T in this word actually sounds a lot like a Japanese/Spanish (I'm guessing there are more)
R.
It works if it's got that slight roll.
The English would pronounce it like a simple T though.
ell.stackexchange.com/users/34393/Synai
I was very impressed when my teacher told me what "beautiful" word pronounced like  beauRiful  and not  beauTiful .  I looking on the internet and everyone pronounced like  beauTiful .  Where I can actually listen to the pronunciation of that word?
How to pronounce the word 'beautiful'?
ell.stackexchange.com
<pronunciation>
ell.stackexchange.com/questions/13040
ell.stackexchange.com/users/1935/Mediator
Your teacher is mistaken. It is pronounced with the "T" sound rather than the "R" sound.  It is also possible that the pronunciation is where the 't' is a merely a glottal stop (common in British English, especially in the London area), or where it has been transformed into a 'd' sound (which is more likely the American version). It is pronounced with 't' rather than 'r'. Sometimes a glottal stop is used.
It is pronounced with 't' rather than 'r'. Sometimes a glottal stop is used.