[susan53]

Sorry - there are far too many questions here to give a detailed answer with a full explanation to all of them. There's a quick answer below, but if you want more information, why don't you take them one at a time and post them separately ? Here goes - your question in normal print, my answer in italics:

1. How much to buy it? -->No verb. Is it correct? Could be - as an "elliptical", or shortened form of the full sentence "How much would it cost to buy it?", in a context such as "A : I'll rent it to you for $45. B: OK - and how much to buy it?"

2) How much for buying it? --> No verb. Is it correct? No

3) It is 3 minutes faster. --> Correct - faster is an adjective.

It is faster (for) 3 minutes. --> Incorrect. Correct : It is faster by 3 minutes

4) How much (for) money/your car? --> Incorrect. Correct : How much for your car? Again an elliptical version of How much do you want for your car?

5) There are 3 more % fibres. Incorrect
There are 3% more fibres. Correct
There are more 3% fibres. Incorrect

6) What the hell is it? --> Correct
What is the hell? --> Incorrect
What nice/happiness is it? --> Incorrect
What is it happiness? --> Incorrect
Why nice/happiness is it?Incorrect
Why is it happiness? --> Incorrect

7) I had never been Australia before but (now) I have been Australia (for 5 weeks). --> Is this correct tense? No. I had never been to Australia before but I went for 5 weeks at Christmas

8) I’m 20 year olds --> Incorrect. Correct : I'm 20 years old.

9) I never thought Correct with changed spelling - eg I never thought about it before
I never think/smoke/do Correct - a permanent fact
I have never smoked/thought--> Correct - at no time in the past up to the present
I had never smoked/thought... Correct : Before a certain point in the past. Until I was 25, I had never thought about getting married.I never did

10) I did it for 3 hours. A past event - eg yesterday
I (always) do it for 3 hours. A permanent fact - eg every Monday
I had done it for 3 hours before I realised what the problem was--> A past event happening before a specific point in the past.

11) I did it from last year to yesterday. --> Correct. "Since" is incorrect here.

I had done it from/since last year to yesterday. --> Again, "since" is incorrect. The whole sentence is unlikely

12) Have to/need/want --> "I have/need/want to see you tomorrow" is fine

13) I had always been scared. OK - before a specific point in the past : "I had always been scared of dogs until my sister bought a labrador puppy"
I always was happy --> Incorrect word order "I was always happy when I was with my grandmother" - a past event.

14) It is a course in CIT that starts tomorrow. Possible if you're contradicting someone -"A : There's a course in BGT starting tomorrow. B : No - it's a course in CIT which starts tomorrow. Not BGT"
It is a course in CIT starting tomorrow. --> Incorrect, but OK with "There's". See the example above

15)I thought that there is there is nothing in this room Incorrect
I thought that there is there was nothing in this room--> Correct