22. I can see that ‘Представь’ is a form of the verb ‘Представить’, but I can’t see what form. It looks almost like it could be an imperative, but it hasn’t followed the spelling rules illustrated in my textbook.
It's a regular imperative, as Боб Уайтман said. I'm not sure how you think it ought to be spelled, but I assume that you were expecting представи, представите? But remember the general rules for forming imperatives:

1. Look at the 3rd-person plural present (for imperfectives) or future (for perfectives), and then remove the verb ending (which will be either -ут/-ют or -ат/-ят) The 3rd-pl. for представить is "они представят", so when you take off the -ят you're left with представ-, ending in a consonant.

1a. If the 3rd-pl. ends in a vowel after you chop off the -ут/-ют or -ат/-ят, then you stick on a -й to form the singular imperative. So они читают gets chopped down to чита-, and the imperative is читай!

2. If the chopped-off form ends in a consonant, like представ-, you next look at the 1st-singular form to see where the stress is. In this case it's я представлю -- which is to say that it's stressed on the verb's stem, and not on the ending. (The fact that the -в- changes to a -вл- is of no consequence as far as forming the imperative goes.)

3. Since this verb is stem-stressed in the 1st-sing., you stick the soft-sign -ь onto представ-, and that's the singular imperative: представь! But if the verb is end-stressed in the 1st-sing., you stick the vowel -и on to make the imperative. So, the verb держать becomes они держат in the 3rd-pl. (stem-stressed), which gets chopped to держ-. But the 1st-sing is я держу (ending-stressed), so the imperative is держи! (and NOT держь!). Again, it's of no consequence that the verb has shifting stress -- you only have to consider the stress in the "я" form.

P.S. Other than that, I can't improve on anything Боб Уайтман wrote!