2. Past perfect tense: the action happened in the past and was completed in the past. That is, it satisfies that 1) actions occur in sequence, and 2) the last one occurs in the past. In addition, in the subjunctive mood, if the subjunctive action involved happened in the past, the past perfect tense should also be used. When she came back, the house had been cleaned. If I had known about it, I would have come as soon as possible.
3. Modal verbs. Can/must/should followed by have done means that you could have done it in the past but didn't do it/must have done it/should have done it without doing it, which is impossible for auxiliary verbs. . .
It depends on your understanding of things. First of all, for material nouns, generally speaking, if their measurement methods are uncountable, they are uncountable. Secondly, the concept of abstract nouns, such as some ideological phenomena, can't be counted if it only stays at the spiritual level, that is, it doesn't refer to anyone. But once there is a modifier in front, or something concrete (that is, the concretization of abstract nouns), you can forget it. For example, success can't be counted when it is said, but if we say that he is successful, then we are talking about "he". With a clear reference, success can be counted. Similarly, the word weather, with or without modifiers, is generally uncountable, but when we talk about all kinds of weather, we can say that in all weather. In fact, there are different kinds here, so let me add that in uncountable material nouns, once different kinds are involved, the plural may be used. However, this has little to do with distinguishing whether a word is uncountable or not, because at that time, the basic meaning already existed. Such as water, water, water.
5. the first one uses a, and the second one uses a. Consider whether there is a problem with the answer. The first is a general reference, which means there is a relationship, but it doesn't mean there is a relationship. The second result is fixed usage. As for A and the, you should remember that except for some fixed usages, the is not needed unless it is explicitly pointed out, that is, the is "definite" and concrete.
6.of means belonging, more of a subordinate relationship, and about means what its content is. For example, we can't say pictures of trees ... because pictures are not trees and trees can't exist. We should talk about pictures of trees ... so when you want to use of, you'd better say whether it belongs to individuals. In addition, sometimes we will talk about the difference between a photo of Lily and a photo of Lily. Here, I hope everyone will remember that the previous sentence said that this is Lily's photo, which belongs to her, and the person in the photo is also her. However, if it is the latter sentence, although the photo belongs to Lily, we don't know whether it is a frog dinosaur alien, and the latter sentence is a simple subordinate relationship. If you want to use this structure to express only "who", remember that it is the latter, that is, you can only say a friend of my father, not a friend of my father (this is like the general exam), which is very troublesome to explain, but you must remember.
7. First of all, it is clear that both much and little can be used to modify the comparative degree. So it's right to choose one point. Then it's time to study why not use rather: it can be used in comparison rather than comparison, because there is a saying that Rather (...) is better than ... . Instead of. . . ) Probably to avoid confusion, rather is not used in this structure. . . I can only tell you this. . . I'm sorry, because this kind of topic is mostly conveyed by language sense, and I almost forgot why. . .
By the way, as for the one that uses a or the, I think it is really possible that there is a problem with the answer, or this question is an exception among exceptions, so don't think much, because it is really rare to use it according to the answer. . . It would be great if what I said could be useful to you. I have been calling for a long time. . . I have a headache. . . Thank you for letting me review my grammar, whether it is adopted or not. . . I almost forgot. . .