When do I use これ, それ, and あれ vs この, その, and あの to say "this" or "that".
example: why is it それはペンです vs そのはペンです?
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これ・それ・あれ act as pronouns, while この・その・あの act as adjectives.
There is also the matter of spatial reference. これ・この refer to things close to the speaker, それ・その refer to things close to the listener, and あれ・あの refer to things away from the speaker and the listener.
Since この・その・あの act as adjectives, you must attach a noun to them. Since は is not a noun, そのはペンです doesn't make sense. An example in which その makes sense would be
それはペンです makes sense, especially if it is a pen near the listener.
The choice depends on whether a noun is attached.
So このペン (this pen) is okay but これペン is not grammatical.
Conversely これ (this) is okay but この is not grammatical.