English Poetry: when to pronounce -ed

[Sel de Guérande]

Hello,

I am attempting to understand when to pronounce the -ed endings of past tense verbs in pre-20th century English poetry.

Authors will often write their past tense verb endings with either -ed or 'd or 't, signifying the word ought or not to be pronounced with an extra syllable.

Tennyson : Match't with an aged wife I meet and dole (...)

In this iambic pentameter verse, the stresses (in bold) would be as follow : Match't with an aged wife I meet and dole (...)

When an author does make this distinction, does this signify all -ed endings must be pronounced as an additional syllable ? or only certain ones ? and on what basis do we make the distinction?

If I understand correctly, we ought to pronounce the -ed ending as an additional syllable (against conventional usage) if the preceding and following syllables are both stressed (in order to maintain the iambic meter)?

Thus in the following verse by George Gordon Byron (from Childe Harold's Pilgrimage):

Richly caparisoned, a ready row
Of Armed horse, and many a warlike store (...)

(stressed syllables are in bold)

the -ed of the word "armed" ought to be pronounced in order to create an additional syllable.

However in this other verse from Byron (from Childe Harold's Pilgrimage):

We once have loved, though love is at an end (...)

we would not pronounce the -ed of the word loved as an additional syllable, since it is not necessary, unless the author wants a spondee ( ... -ed though love) instead of an iamb ? but then why ? how do I infer how classical poetry ought to be read/recited ?

Please help me understand this. I have already asked two English literature teachers and none could answer me - confused about English metrics.

[José]