Certainly! Let's complete each sentence with the appropriate verb forms, and I’ll explain the choices made.

1) I'm soaked to the skin! If only I **had brought** an umbrella!
- Explanation: "had brought" is the past perfect tense used for expressing regret about a past action that did not happen.

2) This pullover was cheap. I wish I **had bought** two of them!
- Explanation: "had bought" is used here to express a wish about a past decision—regretting not having bought more.

3) I like your school. I wish I **could go** there too.
- Explanation: "could go" expresses a wish for a present or future possibility that is not attainable.

4) I must get in touch with Sue. If only I **knew** her phone number!
- Explanation: "knew" is the simple past tense used to express a wish about a current situation—here expressed as a hypothetical.

5) This bus is really slow! I wish we **had taken** the train.
- Explanation: "had taken" is the past perfect tense expressing regret over a past choice.

6) I'm disappointed in this camera. I wish I **had not bought** it.
- Explanation: "had not bought" is the past perfect negative form, expressing regret for a past decision.

7) I answered three questions well. If only I **had finished** the whole test!
- Explanation: "had finished" is the past perfect tense, used to express regret about not completing the test in the past.

8) I can't understand Marie! I wish I **could speak** French.
- Explanation: "could speak" expresses a wish for the ability to do something in the present, indicating a lack of that ability.

In summary, most of the verbs used in the first half of these wishes express regret about past actions or decisions, while those in the later sentences express current abilities or desires that are unfulfilled.