Difficulties in finding of "s third person singular" in more complex sentence

Question

I know the question I'm asking is so specific that you may be reluctant to answer, but I have no other place to find my answer.

I have a sentence, on the Sciencealert site, in which the writer did not use, as my guess, "s" for the verb in the second clause. Why? Is it grammatically correct? How it formed?

The sentence is:

The goal of this study was to search for genetic underpinnings of male sexual orientation, and thus ultimately increase our knowledge of biological mechanisms underlying sexual orientation.

Why the "increase" verb has not s? Is not its subject "the goal"?

Answer 1

"increase" there is grammatically correct because it is infinitive, not present:

• The goal of this study was to search for genetic underpinnings of male sexual orientation and [...] (to) increase our knowledge of biological mechanisms underlying sexual orientation.

It would make no sense to say that "the goal of this study increases our knowledge..." We could say that this study increases our knowledge. Instead, goals are usually expressed with an infinitive after the verb be:

• The goal of this study is to increase our knowledge...

• The goal of this study was to increase our knowledge...

Since the first part of the goal, "to search for genetic underpinnings," is a full infinitive, there's no need to use "to" for the second infinitive. Actually, a bare infinitive (without "to") sounds much better after the linker "thus."