they’ll probably toss it away thinking a freshman crush has no relevance.

I found it in the book “13 Reasons Why”.

Is this grammatically correct?
I want to know how to use gerund (present participle?)= thinking a freshman~.
I can put such a sentence after the completed sentence anytime?

See a translation

It is. Think of "thinking a freshman crush crush has no relevance" as the explanation for why they'll probably toss it away.

Also thinking here is not a gerund. This is a good example of a gerund, "I enjoyed his singing." See how I'm using a verb (singing) as a noun? That's how gerunds work 😸

Another way to look at it is

They'll probably toss it away, thinking a freshman crush has no relevance.

or

A: They'll probably toss it away.
B: Why?
A: Because they think a freshman crush has no relevance.

Did that help? If not, let me know and we'll think of something else~