Please correct these sentences made using conjunction

1. What time does your school start?
2. What time does your school finish?
3. Open your umbrella or you will get drenched.
4. He is very poor. So much so that he? cannot fulfil his basic needs.
5. I was so impressed by his personality that I wrote a book about him.

1 - 'Hi there' and 'hey there' mean exactly the same thing.

2 - Both these expressions are very informal and very common. They can be used when greeting a complete stranger or an old friend. 

3 - The reason you see people using 'hey guys' when recording a video is because they're addressing an audience. 'Guys' is a plural. You could use this in real life if, for instance, you were greeting a group of your friends. 

4 - There are many ways of greeting someone in English. You have probably already meantioned the most common. Beyond that, it really depends on where in the English speaking world you are. For instance, in the UK, one of the most common greetings is to simply say "alright?" - short for 'are you alright?". In Australia they say 'g'day' - short for 'good day'. Apparently.

5 - You mentioned 'how are you?" as a greeting. 'How are you feeling?" is simply the longer form of this. However, take care using 'How are you feeling?' as a greeting because it implies concern - for instance if the person you were talking to was - or had been - ill.

My exciting trip to Novosibirsk (part two)

The city center was also interesting. There were lights, a christmas tree, frost and the biggest flakes of snow. What could be better? Exactly - a hot cup of coffee. I liked three things in Novosibirsk city center – the Opera Building, the history museum (please have a look at the photo attached) and the Chapel of Saint Nilokolay. The Opera Building was constructed during the time of the Soviet Union; nevertheless, it has an interesting architectural style. The history museum reminded me of the truck in the Coca-Cola Christmas advert. The Chapel of Saint Nicolay was the geographic center of Imperial Russia.
I spent two unforgettable weeks in Novosibirsk. At the end of my trip, the weather became cold. I remember the day when the thermometer registered minus forty centigrade. It was extremely cold, I thought that I had frosted my nose (frostbite on my nose?). (No joke) Please don’t believe anyone when they tell you that climate in Siberia is very mild and dry and that you won't feel frosty and cold - they're lying! I felt very frosty and cold.
So I resume (conclude) that Novosibirsk is very nice city but not in winter.

yes, perfectly correct.