I was solving exercises from the Genki II workbook when I stumbled upon this sentence: ”私は先生に東京の大学について聞きました”. It had to be changed into a sentence using humble expressions. There appear to be 2 verbs つく and 聞く, so I thought that both of them should be changed. However, according to the answer key, only the latter one should (to お聞きしました/伺いました). It made me wonder if in sentences with multiple verbs only the main one is changed to the humble speech, and if there are any additional things one should know when making a complex sentence using it (though I might be simply misunderstanding the original sentence).
The textbook doesn't appear to talk about this subject, and I can't seem to find an answer using google (maybe my google-fu is just too weak).