When is "marked" pronounced with 2 syllables?
I have heard "marked" pronounced with 2 syllables like "mar-ked" but online dictionaries show only the 1-syllable pronunciation.
When should it be pronounced with 2, and is it a mistake to use swap their use?
What about the word "aged"?
Marked only has two syllables in poetic or archaic usage.
Aged has two syllables when used as a noun (some of the aged need motorised shopping trolleys), or as a "standalone" adjective (an aged relative). It's only one syllable when used as part of a compound adjective (middle-aged relative), or as a verb (I've aged a year since then).
Some words occur in "set phrases" where the extra syllable is effectively part of an archaic contruction (blessed are the meek).
EDIT: Per John Lawler's answer, and comments to mine and his, the word "aged" seems particularly weird. Some people use the one-syllable version for all contexts, but for those who do use the two-syllable version, the precise boundaries as to where this is appropriate seem somewhat hard to pin down.
Speaking for myself, I read this usage of an aged map as being the one-syllable version, but if my "mental lips" were moving while I read, I would say this one as "an agéd map". I can't easily articulate the distinction, but the agéd version seems more appropriate to people, or where the attribution of antiquity implies venerable rather than old and tatty, ravaged by time.
In the case of aged cheese, wine, etc, they're not normally that old anyway - the word just means they've been matured for the appropriate length of time, not that they are ancient.
I believe, contrary to the above, that the two-syllable pronunciation is quite common in descriptive science in general and, in particular, medical enunciations of symptoms or side-effects of a condition or medication. E.g., "topical application of X is known to cause 'mar-ked' erythema immediately surrounding the area of application".
I suspect your confusion stems from the fact that markedly is always pronounced as a three syllable word. There's no euphonious way to say it with two syllables, so it hasn't undergone the syllabic reduction common for many -ed words in Modern English.
Marked alone is one syllable except in archaic/poetic usage, just as FumbleFingers has already ably demonstrated.
Aged with one syllable seems to be limited to phrases with a number of years
- A man aged 46 was arrested yesterday.
and when referring to non-human things (aged cheese, well-aged beef, unaged wine).
Generally speaking, adjectives formed from regular participles, like wanted, believed, added, whispered, etc, follow the pronunciation rules for the {-ED} past tense morpheme. {-ED} is pronounced
- /-əd/ after dental stops /t/ and /d/ (because it's impossible to say final /-td/ or /-dd/)
- /-t/ after other voiceless sounds /p k f θ s ʃ/
- /-d/ elsewhere, including after vowels
In the case of aged, the final sound is voiced, and therefore /-d/ is possible, but it's an affricate /dʒ/ formed from a dental stop, so the epenthetic shwa is possible, and occurs in some circumstances, as FF notes in his answer.
In the case of clothed (clothed all in white), either /-d/ or /-əd/ is possible because /ð/ is voiced but also dental (final /-ðd/ is hard to pronounce), though the two syllable version is usually marked with an accent to distinguish it (clothéd all in white). See Pratchett & Gaiman's Good Omens for some examples.
In the case of marked, I think Jonathan has it right -- markedly gets the epenthetic shwa to separate the /-ktl-/ cluster, with the /t/ voiced to /d/ by the preceding vowel.
The online Merriam-Webster dictionary gives the two-syllable pronunciation in audio. This pronunciation goes with the second definition:
2 \or ˈmär-kəd\ : having a distinctive or emphasized character ⟨has a marked drawl⟩
(for some reason, this pronunciation doesn't show up on my screen, even though I was able to paste it into this window.)
I definitely remember this pronunciation being used for things like "a markèd limp" when I was younger (maybe 40-50 years ago), but it was rare even then and I would say that this pronunciation is almost obsolete now, even for this meaning, and that there is no reason for you to use it.