Just accept it. If you are speaking to a Russian he most likely wouldn't speak as clear as you want it (not because he is russian but because native speakers don't speak like we learn a language in school :-D) Btw I think it is understandable what ue says - most of the time ...
I am not sure as I have only just started learning Russian, but I think this could be the reason: эта is the fem. Sing. form of the demonstrative pronoun Этот (this/that). It changes according to the number, gender and case. Это (this/it) is indeclinable. E.g. Это новый дом (this/it is a new house); Этот дом новый (this house is new) I don't know if I am right. Hope a native speaker can provide a clarification.
It is more like, we treat "this" differently depending on whether it really modifies a noun or stands on its own. As a rule of thumb, try replacing "this"(these) with "my" or "your". If you have to use "mine" or "yours" in English, it means the pronoun is on its own.
To give you a word-for-word translation:
- This is my computer → Это мой компьютер
- This computer is mine → Этот компьютер мой.
- It was a horse. → Это была лошадь.
- This horse was black. → Эта лошадь была чёрной / чёрная.
When you means "this one", you also treat этот as a pronoun that agrees with the gender/number/case of whatever you decribe. This case is sort of special but it does pop up occasionally:
- Вы двое — со мной. Этот остаётся здесь. You two go with me. This one stays here.