The oven has been turned off for an hour and thirty minutes.

Phrase: turned off for an hour and thirty minutes
Verb: had been
Subject:oven

The secretary and the accountant walked around and found the missing notebook in the trash can.
Verb: walked
Subject: secretary and accountant
Phrase: around and found the missing notebook
Please check my answers.

Both are wrong.

What parts are wrong? Is it the phrases or the verbs?

I think that for sentence number 2, there might have been two verbs. The verbs are walked and found. If the double underlined walked and found, do I just cross out the word around and and?

1 and 2 The subjects are right.

Study this site very carefully to learn about prepositions and prepositional phrases.

http://www.chompchomp.com/terms/preposition.htm

Your answers are almost correct! Just a small correction:

Verb: found
Subject: Secretary and accountant

Phrase: walked around and found the missing notebook

Here's the corrected version of your sentence:

The secretary and the accountant walked around and found the missing notebook in the trash can.

Great job!