For example, there are sayings like “this house is better than yesterday” or “yesterday was so snowy.” Are a thing and spacetime separate things or is a thing a property of spacetime?
Your question is largely generic. Most philosophical texts provide precise definitions of thing, object, entity, system, related to the matter of discussion, so you might find different, and even contradictory answers.
Here are some common definitions:
- object is something that is not the observer (subject);
- entity is used in an ontological sense (so, an object that exists in some context);
- thing is usually something related to space and time, or also a natural entity;
- system is a set of interrelated parts (objects), that is, existing in virtue of its parts, and in virtue of its relations.
So, for your question, the answer is no. A thing is not a property of spacetime, like an electron is not a property of an atom. Things would occupy space and time, according to the previous definition. In such case, a thing is not a property of spacetime, is not a separate entity. It just extends through space and time.
For Immanuel Kant, space and time don't exist in nature, but in our heads, we kind of create them in order to approach nature.
One perspective is substantival dualism, according to which objects are distinct from space time. Material objects occupy regions of space time.
Another is supersubstantivalism, according to which objects are identical with space time.
Stanford Encyclopedia of Philosophy provides discussion of both. (See 4.5 for supersubstantivalism).
Here is an Argument Against Dualism, but it also provides perspectives of both arguments.
There appears to be no consensus.