An English verb has three principal forms: infinitive (forms like write, break and forget), past tense (forms like wrote, broke and forgot) and past participle (forms like written, broken and forgotten).

Read the sentences given below:

The dog bit the man.
The man was bitten by the dog.

Accident and Incident

Accident is something that happens unexpectedly. Note that accidents are usually unpleasant. Incident means ‘event’.

Ten people died in the accident.
It was a strange incident.

Read the following sentences:

Who are you?
Whom do you want to talk to?
Whose is this umbrella?
Of whom are you speaking?
What do you mean?

It is a third person pronoun in the singular number. It has several uses:
a)    It can be used to refer to lifeless things and animals
I have bought a table. It is made of rosewood.
The dog is hurt; it should be taken to the hospital.
I liked that bird because it had beautiful feathers.
Bring the report. I want to see it.

Read the following pairs of sentences:

I saw a little girl. She was crying.
This is the book. You have been looking for this.

Demonstrative Pronouns

Read the following sentences:

This is the bag I bought.
That is a tree.
These are the books I read.
Those are the letters to be posted.
Both pots are big, but this is bigger than that.
He then narrated his experience. That was unbelievable.