Yakusoku

Yakusoku: (1) promise (2) pact (3) engagement (4) arrangement

約束  -- n.

Hint: the kanji 約 is literally means 'promise'. Easy to remember.

Anime hint: anyone remember this song from Escaflowne? The title is Yakusoku wa Iranai. Can you guess the meaning?

Yami

Yami: (1) dark (2) darkness (3) shady (4) despair

闇  -- adj-no, n.

Hints: there's a manga called Yami no Matsue ( 闇の末裔 )

Yasashii

Yasashii: (1) kind (2) tender (3) gentle

優しい -- adj-i


Yasashii: (1) easy (2) simple

易しい -- adj-i

Hint: see the difference between 'yasashi' ( 易しい )  and 'kantan' at this link, which also provided you best explanation and examples.