Learn English – Inspiration to VS inspiration for

Perfect constructions speak about a state which obtains at Reference Time—the time which your discourse addresses—as a result of some prior event. Present perfect and past perfect address different Reference Times.

Present perfect speaks about a present Reference Time:

I am about to start my first novel, but I have been writing since childhood. ... This speaks about my present writing experience.

Past perfect speaks about a past Reference Time:

In 2003 I started my first novel, but I had been writing since childhood. ... This speaks about my writing experience in 2003.

You may read more (much more, possibly, than you want) about the meaning and use of perfect constructions here.

When referring to any sort of jeans or pants in English, one of them is always "a pair". No fluent American English speaker says "a bluejeans" or "a pants".

You can say, "I wore a pair of bluejeans." Or you can say, "I wore bluejeans." But you can't say -- WRONG -- "I wore a bluejeans."

This is true for any clothing worn over the legs that has separate tubes of cloth for each leg, even though they are connected together. "A pair of pants ..." "A pair of shorts ..." "A pair of panty hose ..." Perhaps weirdly, it is also used for clothes that don't have separate legs, like "a pair of underwear" and "a pair of panties". But it is not used for skirts or dresses: We say "a skirt". "A pair of skirts" would mean two separate articles of clothing. Which is kind of funny when you think about it, but whatever.

If, as @oerkelens says, the usage is different in India -- I wouldn't know.