I like to study German using Jenny’s materials at, Learn German With Jenny’s [1]. She explains very well and have many many examples! Sometimes I need to watch it several times to understand the content in full, but this is because it’s complicated and it’s difficult to memorize it actually. But I also use [2] for deep explanations.
So today in the morning I was reading a book, , and I found this zu lost in the sentence and I think that’s the reason

The example below is from [2]. {From what I understood.} In infinitive clauses, it’s basically implied the grammar subject and therefore the verb can not vary! We need the first clause to understand the context and deduce it’s meaning. {I always put From what I understood because it might be a little different in the end}

Es wundert mich, meine Mutter hier zu sehen.

It surprises me to see my mother here.

The second sentence depends on the first, but they are combined in the infinitive form. The infinitive clause is in the infinitive because of the reasons already explained above.

[1]: https://www.youtube.com/watch?v=HwxJilMaCuU

[2] http://www.dartmouth.edu/~deutsch/Grammatik/WordOrder/Infinitives.html