Double-click on any word and see its definition from Cambridge Dictionaries Online.

1. What's the difference between am I and aren't I?

The verb to be is the present simple is formed as follows:

I am / I'm not / Am I? / Yes, I am / No, I'm not.

			Am I the lucky one?
			Am I happy? Yes I am.

However, in question tags, the form for I am is aren't I.

			I'm next, aren't I?
			I'm the one you're after, aren't I?

Note that the form for I'm not in question tags is am I.

			I'm not keeping you, am I?
			I'm not doing it for the money, am I?

2. What's the difference between must and have to, when the obligation is imposed by the speaker, or by others?

If I say

I must start getting up earlier.

I mean that it is me who thinks it is necessary. In this case, in British English, we would not normally use have to. In American English, however, have to is more common.

If I say

I have to be at a meeting at 10 o'clock.

I mean that someone else (my boss?) has told me it's necessary. In this case, in neither British nor American English, would we normally use must.

3. How do we use the word and in long numbers?

In British English, and is always used before the tens in a number.

236,874 = Two Hundred and Thirty-Six Thousand Eight Hundred and Seventy-Four.

Notice that the first and is used before the ten thousands.

In American English, and is usually dropped.

236,874 = Two Hundred Thirty-Six Thousand Eight Hundred Seventy-Four.

4. What's the correct use of the word shall?

Shall is the first person singular and plural form of will:

I/we shall

You/he/she/it/they will

In British English shall is still used, but is becoming less common. In American English shall is not normally used. Probably the area where shall is most common (especially in British English) is for making offers and suggestions and asking for decisions and instructions:

			Shall I make some tea?
			Shall we visit the neighbours?
			When shall we have your answer?
			How shall we find you?

http://www.wsu.edu/~brians/errors/shall.html

5. What form follows the phrase "I look forward to …"?

Look forward is one of a small number of verbal phrases that are followed by to and the -ing form:

			I look forward to hearing from you soon.
			We are looking forward to seeing you at the reception.

Other verbal phrases that take this form include:

To be/get used to
To be/get accustomed to
To take to
To object to

			I'm used to getting up very early.
			He found it hard to get used to driving on the left-hand side of the road.
			We didn't take to being followed everywhere we went.
			We object to paying before we know what we'll get.

6. What's the difference between got and gotten?

In British English, the past participle of the verb get is got:

We've finally got rid of the computer virus.

In American English, the past participle of the verb get is gotten:

We've finally gotten rid of the computer virus.

7. What does the word still mean?

Still (as an adverb) is used to say something is in the present, not the past. We may use it to stress that we are surprised it has not finished.

			What? You're still here?
			They're not still talking about the same thing, are they?
			I'm still happy even though I've lost my job.

Still (as an adjective) means "not moving; staying in the same position"

• Children find it difficult to sit/stand still for very long.
• I can't brush your hair if you don't keep still.

http://dictionary.cambridge.org/

Still (as a noun) means "a piece of equipment used for making alcohol".

http://dictionary.cambridge.org/

Still can also be used as a discourse marker (a word or phrase which demonstrates the connection between what is going to be said or has already been said). In the following:

"My car's very old. Still, it gets me from A to B."

still introduces a point that contrasts with the previous point.

8. Which nouns form irregular plurals?

http://www.gsu.edu/~wwwesl/egw/crump.htm