This is my comprehension after reading your post:

The first sentence implied some uncertainty of going out or not, and whether to close the window or not depends on his going out, whereas the second one is of certain that the person will go out in a future time and thus he'll close the window when he's out.


I hope I don't get you wrong. Thank you, Miriam.

A: "Has he gained weight?" B: "He would gain weight, but he _____ much." (1)doesn't eat (2)didn't eat

The asnwer is (1).

But I chose (2) because I think it is the first condition. In true, he doesn't eat much. So the clause following 'but' should be in subjuntive mood.

A: Tell the nurse you've been waiting an hour. B: Not now...She's talking to a patient. _________ (1) Interrupt her would be rude. (2) It would be rude to interrupt.

The answer is (2)

I want to know what's wrong with (1). Should I make the verb a gerund?

Hello, Pastel.
I'll try to answer your questions; let me know if you need clarification?

Your first question:
A: "Has he gained weight?"
B: "He would gain weight, but he _____ much."

(1)doesn't eat (2)didn't eat

Without a context, with the short exchange as it is, the right answer is 1). B is, in the end, saying that "he" hasn't gained weight: he doesn't eat much; he would gain wait if he ate more.
Now, if you choose 2), we would have a problem. ~L~
There are few cases when you can use a conditional in both clauses in a sentence -a conditional sentence. But your sentence isn't one of those, not in form.
Also, "didn't eat" won't have "subjunctive" meaning in your example.


Your second question:
A: What happened to the flower I gave you?
B: The flower _____ well, but I did not water it.

(1)would have grown (2)should have grown

1) is correct.
If you chose 2) there would be a contradiction in meaning in both clauses: I didn't water the flower, but on the other hand it "should" have grown well? It's not possible, it would have been a miracle for the plant to grow if I didn't water it. See what I mean?
The meaning of the sentence (with option 2) is: The plant would have grown well if I had watered it, but I didn't (so the plant eventually died, I suppose).

You could use "should" in a different context: suppose you did water the pland, yet it didn't grow. In that case you could say, for example: "I watered the plant, it should have grown well", meaning it was expected to grow as a result of my taking care of it... but it didn't grow.


Third question:
A: Tell the nurse you've been waiting an hour.
B: Not now...She's talking to a patient. _________

(1) Interrupt her would be rude. (2) It would be rude to interrupt.

2) is ok.
1) is wrong for the reasons you gave. Instead of "interrupt", you could choose between "interrupting" and "to interrupt". Both a gerund and a to-infinitive can be the subject of a clause.


Your last question:
It is strange that you _____ say such a thing.
(1)would (2)should

2) is correct. Here, the construction with "should" indicates subjunctive mood.
I understand your reasoning for choosing "would", but in this case it wouldn't indicte willingness on the part of the speaker, and "should" doesn't indicate there is an order or srtrong suggestion involved. The choice in this sentence has to do with "form", and it is the form what will determine the meaning... if that makes sense?

Miriam