NOUN -- ITS PLACEMENT IN A SENTENCE

 A sentence comprises Subject and Predicate. 

AS SUBJECT AND OBJECT

A noun(or pronoun) is used as a Subject and an Object in a sentence. Predicate includes a finite verb. 

A noun is a doer in case of Subject and a recipient in case of Object. A Noun does the work in case of Subject and receives the fruit of work in case of Object.

For example

 John writes a letter. 

John ,Subject,  is doing some work. 

John helps Smith. 

'Smith' is receiving help. 

'Smith' is Object here.

AS AN ADJECTIVE

A noun can  modify another noun also. It is called a noun adjunct or attributive noun.

 

Like regular adjectives it does not change forms i.e. it does not take comparative or superlative degrees. Example Mango tree, School Bus, Wall paper, Book worm etc.  

These nouns (used as adjectives) are mostly used in singular form e.g.Table cloth, Bed sheet etc. 

These nouns show the purpose e.g. Office table, Water tank etc.

These show type or category : Army man, Sanitory worker etc.

These can be according to origin London bridge, Wall street etc.

These can be according to material used e.g. Paper napkin, Gold ring etc.

These can be according to time Morning walk, Summer vacation etc.

These are according to possession or relation Group photo, Hair pin etc. 

GERUND

'Artificial nouns' i.e.

Gerunds too are used as adjectives. 

For example Running car, Swimming pool etc. 

A Gerund is used as an adjective  with subject as well as Object. 

For example 

A flying squad reached there. 

'Squad' is doing some work. So it is subject case. 'Flying' a Gerund is modifying it.

He stopped the running water.

'Water' is Object  here and 'running' is modifying it i.e. its Adjective.

DIFFERENCE GERUND AND PRESENT PARICIPLE

 Besides a Gerund, a Present Participle also qualifies a noun. For example:

A swimming person feels light.

Here person is a noun and Swimming is a Gerund qualifying it. A Gerund is acting as an adjective. Person is doing some work it is subject. It is a doer.

A Swimming pool is under construction.

Here 'pool' is a noun and Swimming qualifies it. It acts as an adjective. Here pool is not a doer. Pool is not Swimming. Its quality is Swimming. A pool that is used for Swimming. So its adjective 'Swimming' is present pariciple.

SUBJECT AND OBJECT AS DOER

In active voice subject is the doer and Object is recipent. But sometimes object is also doer. Both subject and Object are doers. 

John saw Smith.

John is subject the doer.

John saw Smith singing.

Here both subject and Object are doers. 

Here present Participle 'singing' is used.

Similarly 

John allowed Smith to go.

In this case both Subject and Object are doers. Both subject and object are doing work. Here infinitive 'to go' is used. Sometimes 'to' is omitted and bare infinitive is used. 

John allowed Smith go.

Here are some examples of Nouns used as adjectives.

Light house, House boat, Rock salt, Sun light, Ship yard, Stone wall, Cricket ball, Rock garden, Flower pot, Day light, Book binder,Battle field, Cow dung, Sun burn, Bathroom, Honey trap,