Furtive means to do something by stealth and clandestine means to do someone secretly or surreptitiously. In most instances they are synonyms. Hope this helps.

We use “so” when we are describing an adjective (good, bad, hard, easy, mean, nice). For example, we say “The acting in the movie is so good.” We can also say “The test was so hard”. We can say "She is so mean". "The teacher is so nice". "The dolls are so pretty". "The children are so good". But when we describe a noun (people or things), we have to use “such”. For example, we can say “He is such a good actor”. We can also say “That was such a hard exam”. "She is such a mean teacher". We can also use "such" with plural nouns. For example, we can say "They are such good children". "They are such obedient dogs".

It is probably 'Which is' It refers to something before it in a sentence. E.g. I can see a car, which is black.

They are all very good. Well done. The only one that needs to be changed is: If she can't aggravate me, she won't be happy. (She won't be happy is in the future tense. i.e. 'she will not be happy')

It means that they will not be able to do the things that they are expected to do.