Plural invariable nouns

Hello 

I get confused with plural nouns such as scissors and binoculars. I find it difficult to learn since these are both countable and uncountable nouns in my language. I have two questions to ask.

Why do I use this and is in this first sentence, and these and are in the second one below?

This pair of scissors is really blunt. It just won't cut!

These binoculars are really powerful!

Thanks in advance.