Sure, let's go through each sentence and explain the correct form of the verb:

**6. My house ______ (build) at present.**
- Correct form: "is being built"
- Explanation: The phrase "at present" indicates that the action is happening right now. Therefore, we use the present continuous tense in the passive voice. The structure is "is/am/are + being + past participle."

**7. We would rather ______ (stay) at home than go out on rainy days.**
- Correct form: "stay"
- Explanation: The phrase "would rather" is followed by the base form of the verb (bare infinitive). So, we use "stay" without "to."

**8. Yesterday, when we visited them, they ______ (have) dinner.**
- Correct form: "were having"
- Explanation: The action of having dinner was ongoing at a specific time in the past ("when we visited them"). Therefore, we use the past continuous tense, which is formed by "was/were + verb-ing."

**9. The weather is terrible today. If the weather __________(be) good, I ______ (go) for a walk.**
- Correct form: "were" and "would go"
- Explanation: This is a second conditional sentence, which is used to talk about unreal or hypothetical situations in the present or future. The structure is "If + past simple, would + base form of the verb." So, "If the weather were good, I would go for a walk."

**10. I wrote to my pen pal 2 months ago, but I ______ (not receive) his reply since then.**
- Correct form: "have not received"
- Explanation: The phrase "since then" indicates a period of time from the past until now. Therefore, we use the present perfect tense, which is formed by "have/has + past participle."

So, the corrected sentences are:

6. My house is being built at present.
7. We would rather stay at home than go out on rainy days.
8. Yesterday, when we visited them, they were having dinner.
9. The weather is terrible today. If the weather were good, I would go for a walk.
10. I wrote to my pen pal 2 months ago, but I have not received his reply since then.