請問this後面接名詞的話，名詞前面就不用再加上冠詞了嘛？為何？

普通的可數名詞在「單數形」時，可泛指同類的任何一個，所以需要加上determiner（定詞）來約束。使用「不定冠詞」a/an時，表示同類當中的任何一個(即any one)均適用，使用「定冠詞」the時，意思等同that one（那一個）。即範圍受限。

this/that是「指示形容詞」，加在「不可數」或「可數的單數名詞」前面都有「限定作用」，所以用了this/that後，就不能再使用有限定作用的「定冠詞」或「不定冠詞」。

If you take nouns after word "this", don't you dare have to add an article to the noun before "this" to avoid overlapping.
"this" is an adj, pronoun;
"This" and a possessive cannot be used together to avoid over-lapping.Sometimes the construction used when "this" and a possessive are both needed in usage.
eg:-
What's all "this"?
What is the trouble?
What is happening?
"This" car of yours needs=your car;
These new dresses and shoes of mine=my dresses.
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"this" adj, pron are used when the person or thing is near in space or time to the writer
eg:-
Look at "this" box.
Are these(plural in no.)books yours?
Are these your children
These are better than those
Life in covid19 is difficult these days.----plural
What are these?
"This" one is larger than that----singular
She will be here "this" Monday.----singular
Is "this" what you want----singular