Spinoza was a substance monist. To the question: What is there? Spinoza would undoubtedly reply, “That which cannot be conceived except as existing, namely God; moreover, there’s only one.” All else, for Spinoza, is property of that one underpinning substantive being.
In metaphysics we deal with this question at every possible level or category or context. To hazard a crude example, say we set our vision over a verdant green cast out before us for miles and miles. Within our field of vision, we will identify enumerable happenings, going-ons, to speak loosely and colloquially, events, objects, processes, entities, substantives, properties, to speak technically (under the language of metaphysics).
Spinoza, as a monist, must also be a reductionist. Everything must be reducible, or defined in terms of, the one underlying substance, God. There is, for Spinoza, beneath the family resemblances, essentially disguised, one thing which answers the skeptic of every degree, a philosophical showstopper. Again, that thing is God, a concomitant of an infinity of attributes each of which express an infinite and eternal essence.
All that esoteric talk really runs a mouth-full. What does it amount to in more practical terms? Well, I am really trying to figure that one out. Spinoza would likely, rather than the Realist, turn away from reducing everything to substantives (things, objects) and processes (events, activities). Spinoza held the mode to be the essential manifestation of contingency in reality (or Reality, perhaps). If you want to speak of the contingent matters, the matters of fact, the cats on the mats and the Kings of France, you are getting into talk about the modes of substance.
For us, that just means: the properties of Reality. But we take “property” from the ordinary language all the way down to the speculative-metaphysical tongue. To introduce a model: subjects and objects are properties of substance (say, God or Reality or whatever). So the cat and the mat are properties of something, namely the underpinning reality. Properties, say, like a smile is to a face. We may say mats and cats, in any particular setting, inhere in the world (reality) in the way a smile inheres within a face. The smiling face is not a thing but nor is it an activity or process. The concept of smiling face captures more than its mere physical (substantive and chemical-muscular sequentive) aspect.
But is it merely an event? Well, it certainly is an event to smile. We certainly smile at this or that time. But is that all we get from the smile? “Event” is a gestalt term. It means very little without qualification. But does “mode” service us any better? Whatever these terms amount to, their acceptance certainly is step away from the ghost chase for real entities whom may correspond to the theoretical posits of our modeling.