Does Alexandra always complete her work on time?
It is not specified whether Alexandra always completes her work on time. The sentence suggests that the information about Alexandra’s punctuality is unknown or has not been provided.

Has this group been popular with the young since last year?
Yes, the sentence implies that this group has been popular with the young since last year. The popularity has been ongoing from the past until the present.

When I entered the room, were my parents talking to my teacher?
Yes, when you entered the room, your parents were talking to your teacher. The sentence describes the situation at the time of your entry, indicating that the conversation was already in progress.

Did Mother say we would go to Crimea in summer?
Yes, Mother said that we would go to Crimea in summer. The sentence confirms that the statement was made by Mother in the past.

I cannot talk to you now, but I will ring you up in half an hour.
The speaker cannot talk at the moment, but they assure the other person that they will make a phone call in half an hour. The sentence indicates a plan or intention to contact the other person later.

Are you in a hurry? Shall I drive you to the station?
The speaker is asking if the other person is in a hurry and offers to drive them to the station. The sentence suggests a willingness to provide transportation assistance.

Will we sing «Happy Birthday» as soon as Nick enters the room?
The sentence proposes that the group will sing «Happy Birthday» once Nick enters the room. It implies a future event or plan.

Is the scientific expedition leaving for Africa tomorrow?
Yes, the sentence states that the scientific expedition is leaving for Africa tomorrow. It indicates a future departure.