Dear native speakers/teachers,
In the following contexts I have two questions and would be grateful if you answer them.
In the first context, why "have seen" has been used in the last sentence and not "see"?
1. First of all, let's define train spotting. There are said to be some 100,000 train spotters in the UK. What do they do? Well, exactly as the title suggests, they spot trains, that is, they stand in train stations, look at the serial numbers of the trains that leave and arrive and write them down. The ultimate aim is to have seen every train in the country.
In the second context, why "did" has been used after "so"?
2. As the number of trains grew and they got faster and faster, so did the interest in them grow.
would you please bhaisahab explain more!!!!!!