Hi! Let's see...
> when to use the word tissue, and when to use handkerchief
"Tissue" here stands for "tissue paper" and a handkerchief is made of cloth.
> what to say when somebody sneezes...
When someone sneezes, people nearby may say "bless you". The reply is usually "(I'm) sorry". In other cultures, people say "thank you", but not in English.
> Next day I sneezed on purpose twice but got no reply).
Don't expect people to react the same everywhere. According to new etiquette, nothing should be said after someone sneezes. Remember that saying "God bless you!" started after an epidemic killed hundreds of people in Europe (in the Middle Age I think) for one of the initial symptoms was sneezing.
> what to say if I want a window to be opened, but not completely...
If you want it to be done well, do it yourself.
![Smile [:)]](/emoticons/emotion-1.gif)
You can say "May I open the window slightly?". The expression "Shall I open the window?" means "do you want me to open the window?" If someone offers, ask him or her to do it, but slightly.
> when to say fountain pen, ballpoint pen or just pen...
A fountain pen is one which uses "liquid ink" and are usually refilled manually or with "cartridges" - these pensa are often expensive. A ballpoint pen uses "dry ink" and refills are purchased at any stationary's - these are the most common. If you don't care about the type, you simply call it a "pen".
> I'm feeling cold, please turn on the heating/radiator
A "heating" usually refers to the central system supplied for heating in a building. A "radiator" is a device for heating a room, building, etc., consisting of a series of pipes through which hot water or steam passes (electric versions are also possible).
> ...god bless America and god save the Queen!
Just in case, uh?
Hope this helps!