lemon vs sweet lemon

lemon and sweet lemon both are nouns.

Word	Noun	Adjective	Verb	Adverb
lemon	Yes	No	No	No
sweet lemon	Yes	No	No	No

As nouns, sweet lemon is a hyponym of lemon; that is, sweet lemon is a word with a more specific, narrower meaning than lemon:

lemon: a small evergreen tree that originated in Asia but is widely cultivated for its fruit
sweet lemon: lemon tree having fruit with a somewhat insipid sweetish pulp

Other hyponyms of lemon include Citrus limetta, sweet lime.

lemon (noun)	sweet lemon (noun)
an artifact (especially an automobile) that is defective or unsatisfactory	lemon tree having fruit with a somewhat insipid sweetish pulp
a strong yellow color
a distinctive tart flavor characteristic of lemons
yellow oval fruit with juicy acidic flesh
a small evergreen tree that originated in Asia but is widely cultivated for its fruit