Traditional grammar defines the object in a sentence as the entity that is acted upon by the subject.[2] There is thus a primary distinction between subjects and objects that is understood in terms of the action expressed by the verb, e.g. Tom studies grammar—Tom is the subject and grammar is the object.
https://en.m.wikipedia.org/wiki/Object (grammar)
please explain the use of "there is thus" in the context. What does "there is" refer to ? When do we use "thus"?
g. "Waiter! ").
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"there is [...] a primary distinction" expresses the existence of the distinction (same "there is" as e.g. "Waiter! There's a fly in my soup!").
"thus" means "therefore"; i.e. the existence of the distinction follows as a logical consequence of what has previously been said.