The likely word is cascade.
The word is "classifieds" (classified ads).
There is no one-word substitute in English for "son-in-law."
The word you want is florist.
a waterfall.
The words for a group of islands may be a chain or an archipelago.
A small section of grass is a clump. A wide area of grass is a lawn.
Sportsmanship is a word substitute for spirit of game
The word metalworking is one word.
eat, use
One possible substitute for "their" is "his" or "her" depending on the gender of the person/people being referred to. Another substitute can be "its" when referring to a non-human object or entity.
The word that works is idiot.
Elevate