глухих The most common meaning of this adjective is ‘deaf,’ but it is also the standard way of describing a ‘dense’ forest.
дубровах A poetic term for ‘leafy forest.’ More commonly дубрава.
Монах The monk is identified in this poem in several ways: монах ‘monk,’ старец ‘elder,’ анахорет ‘anchorite,’ старик ‘old man,’ and отшельник ‘hermit.’
лопаткою Nsg лопатка ‘Shovel.’
смиренной ‘Humble.’
рыл ‘Dig.’ Conjugated like мыть.
вожделенной ‘Desired.’ The word is built from the prefix воз- and the root жел ‘desire,’ and expresses a high degree of desire (lust, craving).
Святых угодников Угодник is a colloquial term for ‘someone eager to please,’ but when modified by the adjective святой it means ‘saint.’
порогу ‘Threshold.’
Поникшей Inf поникнуть ‘droop.’ This is a past active participle.
хижины Nsg хижина = изба.
черней The ending -ей is sometimes used in place of the more common comparative adjective ending -ее, especially by poets who need to economize on a syllable.
стал Стать is a common synonym of начать.
села Note the change of tense from the preceding verb, comparable to the mixture of past and present tense in narration in English.
брегов Nsg брег; a poetic variant of берег.
влажные ‘Damp.’
власы A poetic alternative to волосы, useful when the poet needs to economize on a syllable.
со страха A synonym for от страха.
красы Nsg краса is a poetic equivalent of красота. The plural is used here with singular meaning.
манит Inf манить ‘beckon.’
рукою When one gestures with a part of the body in Russian, that body part is expressed in the instrumental; note the same construction when the rusalka nods her head in the following line.
Кивает ‘Nod.’
Под сонной скрылася волной ‘Hid herself under the sunny wave.’ Note that the adjective and noun are separated by the verb and the verb takes an alternative poetic ending in order to meet the required syllable count and meter (the standard form would be скрылась).
Все Elliptical for все время.
чудной девы тень The genitive of possession precedes the thing possessed here, unlike in standard Russian, in order to fit the metrical requirements of the poem. Дева is a poetic synonym of девушка.
тьмою The fIsg ending is normally -ой, but it is often lengthened to -ою in poetry when an extra syllable is required.
шутя Inf шутить ‘joke.’ This form is an imperfective verbal adverb (sometimes called a gerund).
плещется Inf плескаться ‘splash about.’
дитя A poetic synonym for ребенок. This word is neuter, despite what looks like a feminine ending; it has an idiosyncratic declension and is rarely declined
стонет Inf стонать ‘moan.’
тонет Inf тонуть ‘sink.’ In other contexts this word can also mean ‘drown.’
бороду Nsg борода ‘beard.’
седую Russian usually uses серый for gray except for gray or white hair (including beards), when седой is used instead.