I want to master pronouncing contractions to sound more natural. I think I know the technique to pronounce "It'd be", but to be honest, I'm not sure if it makes much difference to add a stop d after it.
So, are they different? Would you notice immediately if somebody said "It be" instead of "It'd be"?
It sounds like you might have been thinking that it'd could be pronounced in one syllable as [ɪtd], but that pronunciation seems unlikely to me. I would also advise against pronouncing it'd in one syllable as [ɪt] (like it).
You can pronounce it in two syllables, with the 'd corresponding to the sound [əd] (references: Blog post by phonetician John Wells; Teflpedia). For me, as an American English speaker, the t of it would be voiced and "flapped/tapped" in this context because of the following vowel sound: [ɪɾəd]. In other accents, the pronunciation with two syllables might sound like [ɪtəd]. The reason for the apostrophe in this case is to represent the absence of the [w] consonant sound, not to indicate that there is no vowel sound between the t and d.
My intuition is that in quick speech, I often pronounce it'd in one syllable as [ɪd]. Wells mentions [ðæd] as a possible pronunciation of that'd. (He also says "maybe ðæt", but my intuition disagrees with him here.)
When a vowel sound follows, it'd could sound the same as it for me due to flapping/tapping. For example, I think my pronunciation of "It'd have been..." could be transcribed as [ɪɾəvbɪn] (which to my ears sounds like /ɪdəvbɪn/). But in it'd be, the following sound is a consonant: in this position, the distinction between word-final /t/ and /d/ can generally be clearly heard.
You could also keep in mind that when a /d/ sound comes before another plosive consonant in English, the /d/ may have its release "masked" by the second consonant, resulting in a pronunciation that sounds like it has the same "place of articulation" as the second plosive. In this case, that would make it sound like [b]. So [ɪɾədbi] or [ɪdbi] could sound like [ɪɾəbbi] or [ɪbbi] respectively. I'd hesitate to recommend aiming for a pronunciation with [bb] as a target, though: I think I've read that some studies have found native English speakers may actually still put their tongues in the position for making a [d] sound in contexts like this, even if it's hard to acoustically detect this [d] before the following [b]. (Nevertheless, some descriptions do call this a form of assimilation.)
In my pronunciation (native North New Jersey), at least, "it'd" is two quick, short syllables; they're almost the same as each other, except a "d" in the second instead of a "t". Like, "it-idd", quickly. Or, if I'm not really carefully enunciating, probably even really indistinguishable from each other, like "iddidd".
I'd be surprised to hear someone pronounce it in a single syllable -- if I noticed.
