1. It's been a long time since my car lost its hubcaps.
"It's been a long time." -- That could stand on its own as a complete sentence. (It is an independent/main clause.)
"My car lost its hubcaps." -- That could stand on its own as a complete sentence. Is could be independent.
However, when you add "since" to "my car lost its hubcaps," that clause becomes subordinate or dependent. It cannot stand on its own.
"Since" is a subordinating conjunction. It joins two parts of the sentence, making one of them subordinate.
"It's" is short for "it has." The contraction can also be short for "it is."
"Its" is the possessive pronoun -- it shows possession. It does not have an apostrophe.
2. Because I won two new backpacks yesterday when I went to the mall, I now have two too many backpacks.-->
"I won two new backpacks yesterday" could be an independent clause (a complete sentence), but adding on "Because" makes it dependent or subordinate.
"I went to the mall" ould be an independent clause (a complete sentence), but adding on "When" makes it dependent or subordinate.
"Two" refers to the number.
"To" is the one you use when you are going to something, or if you want to do something.
"Too" can mean "also" or "excessive, more than enough."