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it would appear that there might be some Americans who don't pronounce the second "f" in "fifth", so it would be more like "fith"
I can confirm that. I lived for three years in a community in western Michigan where *nobody* pronounced the second F in fifth (and they looked at you kind of funny if you did). The resulting pronunciation was *exactly* like "fith". The phenomenon, however, is not particularly widespread.
I am struggling to think of a genuine standardly-pronounced word with a silent f.
I don't think there are any, unless you count cases where the entire syllable containing the f is routinely omitted (e.g., camouflage). That's really not the same phenomenon as a silent letter.
Then again, several of the ones on the list aren't really the normal "silent letter" phenomenon either...
The A in aisle is part of a very standard diphthong that is pronounced the same way in English as in many other languages.
The B in subpoena isn't silent so much as (sometimes) devoiced.
The D in badge *is* pronounced -- you just don't notice it because the letter J (and G when making the same sound as J) has the D sound built in anyway, most of the time. (It's an affricate. If you take away the D portion, you're left with the J sound in the the French name Jacques, sometimes indicated via "zh" in English dictionaries. The S in measure makes the same sound, as does the G in camouflage when that syllable is pronounced. If you prepend D, you get the combination of sounds found in badge. While we're at it, ch is also usually an affricate, with a T sound built in; if you take away the T you get the sh sound; there are some words in which ch makes the sh sound without the T sound in front, but they are in the minority.)
The E in "pie" has exactly the same phonetic significance as the E in "hide" and "bake". ("When two vowels go out walking, the first one does the talking.") Well, almost. Admittedly, "pi" would be pronounced the same way, but only because it's just two letters long. If we add something else on the end, to make e.g. "pied" or "pies", the sound does not change; without the E, it would: "pid" or "pis" would be pronounced with a short vowel.
The L in "would" causes the "ou" to make a very different sound from usual. (Without the L, "woud", if that were a word, would probably rhyme with either "food" or possibly with "hued" or with "rode".)
The O in "opossum" is pronounced, at least in American English; there just happens to be another word "possum" that lacks it, and the poll writer is probably getting them confused. (Compare "until" and "till".)
The Rs in "surprise" are also pronounced in, as far as I know, just about every dialect of English that consistently pronounces the R sound in general. There are several rather prominent dialects of English that routinely leave out the R in words that have it and/or insert it into ones that don't, but the word "surprise" is not special in this regard, to the best of my knowledge.
The second V in "revving" is there to make the E short, since only one V would not be enough to separate it from the I. (Compare "hater" versus "hatter", "later" versus "latter", "latte" versus "late". There are hundreds of thousands of such words in English.)
The Y in "they're" is exactly the same as the Y in "they": it's part of the "ey" diphthong, which makes a different sound than E would do by itself. (Granted, there are a couple of dialects (notably including one that was originally urban street slang and is now common in large parts of the US) wherein the pronunciation of "they're" gets shortened to one syllable and a different vowel sound, thus matching "their" and "there", but that's not standard, or at least wasn't until very recently. The same dialects also pronounce "you're" the same as "your" and "yore", which makes me twitch every time I hear it.)
One that's omitted from the list is the T in tsunami, which has an entirely different linguistic background from the T in depot. Some of the ones on the list have examples that use other letters, e.g., the K in know is exactly the same as the G in gnostic, right down to coming from the same Indo-European root.