Piya has slept only three hours . Which part of speech is the word 'only' here ?

The correct preposition to fill in the blank is across . making the sentence: "The road runs across hill and plain." 

The word 'army' is a collective noun because it names a group of soldiers acting together as one single body. 

The  noun form of the verb "know" is knowledge.

In the sentence "We shall not see his like again," the word "like" functions as a noun. It means an equal, a match, or a similar person or thing. 

The complex form of the sentence is:I knew what his name was .

The negative form of "Akhi missed the train" is "Akhi did not catch the train"