Questions & Answers about Onko tämä pinta lasia?
Why is lasia in the partitive case here instead of the nominative lasi?
Because lasi (glass) is being treated as a material or uncountable substance here. When expressing what material something is made of, Finnish uses the partitive case. If you used the nominative lasi, it would mean "Is this surface a drinking glass?"
If I wanted to say "Is this surface made of glass?", could I use lasista instead?
Yes, you could say Onko tämä pinta tehty lasista? (Is this surface made out of glass?) using the elative case (-sta) to mean "out of". However, simply using the partitive (lasia) with the verb olla (to be) is the most common and natural way to describe an object's material.
Could I ask this by saying Tämä pinta onko lasia?
No, that would sound unnatural. The question particle -ko is attached to the specific word you are asking about, and that word almost always moves to the very front of the sentence. By putting Onko first, you are asking "Is it...?"
Does pinta only mean a physical surface, or can it be used abstractly?
It primarily means a physical surface like a table top or the surface of water, but just like in English, it can be used abstractly. For example, the phrase pinnalla (on the surface) can describe something that is superficial or currently trending in the media.