These are the great lakes. There are more than one great lake and each has a different name. If the WHOLE area is called "the great lakes" then you would use "this". Each lake is known as a great lake, so a collection of great lakes is referred to as "these"
This/these are both right. "This is the Himalayas" refers to the region called the Himalayas. "These are the Himalayas" refers to each mountain in the mountain range as a collection, called the Himalayas
These are the Taylors/Perellis. There are more than one person with the same surname, so you say these. Even though they are a couple, they are still individuals. IF they have become known (been given a noun) as the parellis - then you can use this is. "Brangelina" was a name given to Brad Pitt and Angelina Jolie. In this case, a couple have been given a collective name. This is Brangelina - refers to both. Make sense?
These = more than one.
This = one (including a region - like the Himalayas
2) I think either works well here. I would probably say "this is Song Doong cave". Its a single cave and you don't need to add "the". If there was a region of caves called the Son Doong caves, and you were pointing out the cave that gave the whole region its name, then add "the" - but emphasise "the" - because its THEEEEEE Son Doong Cave that gave the whole region its name .... its important! "Hi, I'm Brad Pitt", " Not THEEEE Brad Pitt?!"
3) The Waitamo Glowworm Caves - more than once cave gets "the"
The use of "the" is diffcult.
The white house
The Doncaster hotel
Houses of parliament
Trump tower - not the
The daily telegraph
National geographic - not the
The Eiffel tower
Sydney opera house
Maybe someone in the forum could help with this one??