First, let's differentiate the differences between archaic words and uncommon words and uncommon usage of words. These are three different things though they may be confused with each other. Archaic words are those that have actually gone out of usage, whereas once they were in common everyday use, and are definitive of another era or time period; they are antiquated.
(1) One way to recognize them is to notice suffixes that are unusual. (2) Another way to find them is to look up suffixes (like -est or -st) in the dictionary; if archaic, the entry will read something like "forming the archaic." (3) Another way is to look up whole words, like "eke." the dictionary entry will say something like: "eke adv. Archaic: Also" (Collins English Dictionary)
Uncommon words are words that are still current but have faded in prominence and are rarely used or heard. Uncommon usage of words generally reflects the application of various rhetorical word schemes to render usage striking and unusual.
In Keats' poem, some of the archaic words are pronouns while others are verb tenses or derivations that are no longer a functional part of contemporary English. The pronouns are thee, thy, thou. Second person formal pronoun is represented by "thee," used to address a person of high rank, for example a minister or a mayor or your teacher. Second person casual is represented by "thou," used to address persons of your own rank or lower, for example, friends and siblings; it is also used from adults to children. The possessive of second person is "thy." It expresses posses ion in various senses: e.g., "It is thy turn to go"; "It is thy rudeness that hurt him"; "Will thou have thy chocolate cake now?"
Word tense (time orientation) usage is represented by archaic watchest and hast. Second person singular past and present tense is expressed in "watchest"; -est indicates something occurring in the past or present moment in second person thee/thou and can be combined with a modal to indicate future time as in "Thou must watchest the roast in the oven for me." Other examples are "Did thou watchest Monday's football game?" (past); "Please watchest thy step!" (present). Second person singular past, present and future combination may also be expressed in "thou hast," which signifies the same as "you have."
Some uncommon words in the poem that are not archaic are garden-croft, bourn, laden, drowsed.
- garden-croft: Scottish; a small enclosed bit of land near a house used for a garden
- bourn: English; small stream
- laden: English; to be loaded down as with a heavy burden to carry
- drowsed: English; past tense of drowse, as in to be half-asleep
Some instances of uncommon usage are "seeks abroad," meaning to look far and wide; "think not of them," signifying "do not think of them"; "lambs loud bleat," which is a word scheme that rearranges "loud the lambs bleat" for poetic effect; and "drowsed with fume," which uses a word scheme that shortens the word "perfume" and signifies "made sleepy with perfume."