an easier way is, you can understand 个 as a measuring word, seperated from 这, the same as how you understand 座
so 这 will be this, and 这个 will be this one
it's not really convincing when you say 这个皮夹, but when you say 这杯咖啡, this cup of coffee, you'll see why you need to put a measuring word between 这 and the noun
that is to say, in a sentence, especially when being a subject, normally 这=这个+noun, if you use 这个 without a noun, that is like you say 'this cup', but not 'this cup of coffee'
you can use them the same as english
let me give you some examples to tell the differences:
i'll take this one. 我要这个
this (this answer) is not correct. 这是不对的/这个答案是不对的 (here 这=这个答案)
this (doing this) is not good. 这不太好/这样不太好/这样做不太好 (normally 这 could be equal to 这样, they both mean doing this)
i'll tell you if i think of more exceptions, but i think the explanation above will work so far :)