Help understanding a grammar excercise about tenses

Question

Which is the correct answer and why?

1. Let us go to the carnival, (shall we / shan't we / should we / shouldn't we)?

2. All but Stella (is being / were being / has been / have been)auditioned for the ballet performance this afternoon.

3. In a year's time, he (is becoming / had become / shall become / will have become) a qualified doctor.

In my opinion, the answer for Q1 is 'shall we'. However, in question tags, the question tag is negative if the main part of the sentence is positive. It doesn't seem to apply here so I'm not sure.

As for Q2, 'All is the subject' but I've no idea what the answer is.

In Q3, I think the answer is 'shall become' which describes the future. 'Is becoming' is unlikely to be the answer as it means 'gradually changing'. I think the future perfect tense 'will have become' is also an answer as it describes a future action that will have been completed.

Answer 1

1. Let us go to the carnival, shall we?

Your instinct about Q1 is correct - "shall we" is the best answer. There are some exceptions to the positive/negative question tag constructions. The Question Tags page on the Woodward English site lists some of them:

Exceptions

Some verbs / expressions have different question tags. For example:
- I am - I am attractive, aren't I?
- Positive imperative - Stop daydreaming, will / won't you?
- Negative imperative - Don't stop singing, will you?
- Let's - Let's go to the beach, shall we?
- Have got (possession) - He has got a car, hasn't he?
- There is / are - There aren't any spiders in the bedroom, are there?
- This / that is - This is Paul's pen, isn't it?

2. All but Stella have been auditioned for the ballet performance this afternoon.

This sentence is a little ambiguous because you have to assume that auditions don't happen on the same day as the performance. If you know that the auditions happened in the past, then you have to decide what the proper verb agreement might be, "has been" or "have been". Understanding the subject is "All" is important and I think you would have picked 'have been' if you knew it was in the past.

3. In a year's time, he will have become a qualified doctor.

This one is a little tricky, because choosing between "shall" and "will" can be hard. Oxford Dictionaries explains that in British English "shall" is used with the first person, while "will" is used with the third person, UNLESS you are expressing strong determination. In that case, the roles are reversed. In American English, "shall" isn't that commonly used.