How would a native speaker connect the /d/ in "I'd" to the /a/ in "advise" when speaking?

To be honest, the first d in “needed” and the t in “city” are pronounced identically to my native ears. But to (hopefully) answer your question, connecting “I’d” and “advise” would sound the same as saying the nonsense phrase, “eye dad vise”.

They're both the same sound, /ɾ/

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American here. I would say the /d/ in both words the same; same if you were fully saying "I would" instead.

The rules are: 1. T or D is flapped [ɾ] between vowels provided that: 2. The following vowel is unstressed Word boundaries don’t matter, so if a T or D at the end of a word is preceded by a vowel and followed by an unstressed vowel, then it would generally be flapped in natural North American speech. There are exceptions for emphasis and some dialects may vary, but most people would flap the D in “I’d” when “advise” follows it: [a͡iʲ.**ɾ**ɛ̞d̚‿ˈdva͡iʲːz̥]. The same is true of both D’s in “needed it”: [ˈniʲ.**ɾ**ɪ.**ɾ**ɪ̆ʔ]. Note: Phonetic transcriptions will vary wildly based on accent. These are based on my own pronunciation.

As far as I can tell, mine is a regular /d/, it's just shoved one syllable over (I dad vise).

This may be regional, but I'd never even consider that the 't' sound in 'city' could take the place of a 'd'. In most of England we don't flap 't's.

This is going to be dialect/accent-dependent, I think. My accent uses the flap t, but I have heard the flap d in other accents.

In my accent (Midwestern USA), saying "I'd advise…" can sound pretty much identical to "I Dad Vize," or maybe even "I Duhd Vize" if I'm speaking quickly. (Vize isn't a word, but it helped me write out how I pronounce the phrase) Given that "I Dad Vize" and "I Duhd Vize" are complete nonsense, though, everyone can catch on that what I'm *actually* saying is "I'd advise."

mostly I just say I would advise. that phrase tends to sound very forceful and demeaning. and only use it in places of authority. I'd advise you to clean up your room or there won't be any supper. versus. Can you please clean up your room,. It's messy and supper is soon. What if I don't? I'll sell you on Craigslist.

It doesn't change the d sound as much as the a sound -- it shortens the a in advise, so it's more like I did vyze

When I say it the sentence SOUNDS like "I dad vise you to" if that helps

Umm, aren't those two "flaps" the same sound?

Depends on the speaker's accent and how fast they're talking. In casual speech, it's often changed to a flap sound, similar to the examples. So, if you're speaking casually, it'd be perfectly natural. But if you're speaking more formally, you might choose to pronounce the /d/ more clearly.