"Schaar" means one PAIR of scissors, and several pairs of scissors are "scharen." You have to look at the verb "is" vs. "zijn". "Is" = one pair = "schaar", "zijn" = more than one pair = "scharen."
In English, "scissors" is both plural and singular, so if they ask you to translate "my scissors are yellow," you have no way of knowing whether they are speaking of one pair or several pairs, so "mijn schaar is geel" and "mijn scharen zijn geel" should both be correct answers to that question. If they mark you wrong for that, report it.
The word scissors is an obligate plural. (Oxford new American Dictionary) You have to use third person plural with them. English has a fair number of these (pants, glasses in the sense of eye glasses, shorts (short pants), pajamas, shears.) A pair of scissors is singular, but the verb is then agreeing with the word pair, not the word scissors. Scissor (without the s) can be a verb or an adjective.