In facing this issue between это and эта (etc), I have become very frustrated. However, my girlfriend gave me some tips.
The relevant tip here is:
If you want to say "This is X" or "X is Y", always use это.
If you are saying "This X", use the appropriate gendered form, in this case эта.
это проблема: this is a problem.
эта проблема: this problem. (Yes, they sound the same, but as someone else said, in speech there would hopefully be more context including inflection)
это офис: this is an office.
этот офис: this office.
это кошки: these are cats.
эти кошки: these cats.
The one time this gets truly conflated is for neutral gendered words:
это яблоко: this is an apple.
это яблоко: this apple.
"X is Y" by the way is the general case of "This is X". e.g. кошка – это проблема: the cat is a problem.
You can practice this by wandering around in the street and pointing at things, and saying "это #, этот/эта/это/эти # хорошо!", and then grinning at passing strangers.