Can someone explain to me why この changes in このペンは誰のですか & これは誰の鉛筆ですか? I was taught that この was supposed to specify a noun so wouldn’t the sentence be このは誰の鉛筆ですか?
これ is the noun version. この can only be used in combination with a noun, not on its own.
You can remember which is which by the この, which is technically the same の you use to connect two nouns, so that one goes with a noun.
I don’t really consider この its own word because of that, it’s more like こ and の. Because of the “の” it must be attached to another word. I think it is possible to omit the word if it’s obvious from context, but it is implied that there should be a word there.
It’s この when it comes immediately before another noun (このペン). If it has は after it, it has to be これは. The first one is like “this pen”, and the second one is like “this is.” In English, “this” stays the same, but in Japanese it changes depending on whether it’s being used as a modifier or a noun.
この is what we’d call a pre-noun adjectival In English. In Japanese, this is called 連体詞. This means that it can modify nouns just like an adjective, but it can’t be used on its own (like the noun これ) and can’t be inflected like other adjectives. Other examples include 大した and 小さな.
これ is this as in “look at this.” この is this as in “look at this book.”
The の in この is the usual particle の.