Neither. You do not use an apostrophe to make a plural. The boys.

I bet your problem is really anxiety rather than your English. If you came to England at 18 it's very unlikely you will totally get rid of your accent. You may feel more confident if you just accept that. It really isn't a problem

No difference in British English. In american Englisg a jumper is a different type of garment - I think what we would call a pinafore.

'till' is not a shortening of 'until'; it is a word in its own right

There isn't one word for 15 days; only 14 days as Manoj states.

no. I overslept for at least an hour.

underslept - no. Overslept means having woken too late (not having had too much sleep).

phew..there's a few problems with that sentence! 'In point of time' is incorrect. It should be 'At that point in time' - meaning at a specific time in the past.

I'm finding the heading, and thus the purpose, of this survey confusing. If you don't know what the survey is about yourself or its purpose, how can you describe it in a title? Can you ask for more information before you do this?

European institutions will usually require a qualification in teaching English as a foreign language; a different skill and qualification to 'English', so you might want to look at doing a TEFL or CELTA course. Other countries, especially