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i would like to know when i should use the Adj before and when after noun because i am very confued . Is there any rules / grammers thalk about that ? As l know that An Adj always become befre a noun .

Fore example

Adj after noun :

# The letter needing immediate anewers are on the desk. (The letter that is needing ...)

(why needing letter is wrong?)

# The film now appearing at the local theater is my favorote. ( the film that is appearing )

(why appearing film is wrong?)

# The train arriving at the sation now is an hour lare. (The train that is arriving )

(why arriving train is wrong?)

Adj before noun:

# The waiting parents were very worried.(The parents who are waiting)

# The crying baby needs to picked up.(The baby who is crying)
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Most adjectives come only before the noun they modify: a beautiful day; an exhausting lesson.

Some can come before of after: The train arriving, the arriving train; time enough, enough time

Some can only come after: The film appearing; the men aboard.

I don't know why that is, offhand. I cannot see any definitive difference in the examples you have supplied, except that the attribute adjectives sometimes seem less 'verbish' than the post-modifying ones.
Comments  
Adj after noun:

But in your first three sentences, the ‘–ing words’ are verbs, not ‘participles as adjectives’. Just because they are participle in form says nothing about their syntactic function. For a participle to be adjectival it needs to function as an adjective, which these do not:

1. The letter [needing immediate answers] is on the desk.

Here, ‘needing’ is a verb (not a participle) because it functions as a verb; you can tell this because it has the direct object ‘immediate answers’. That’s why you can’t say ‘needing letter’.

2. The film [now appearing at the local theater] is my favourite.

Again, ‘appearing’ is a verb (not a participle) because it functions as a verb; you can tell this because it is modified by the adverbial ‘at the local theatre’. That’s why you can’t say ‘appearing film’.

3. The train [arriving at the station] is an hour late

Similarly, ‘arriving’ is a verb (not a participle) because it functions as a verb; you can tell this because it is modified by the adverbial ‘at the station’. That’s why you can’t say ’arriving train’.

(Incidentally, the subject of each of those three sentences contains a reduced relative clause, where the relative pronoun is omitted and the verb is nonfinite).

Adj before noun:

Your next two sentences do, however, contain participles as adjectives: ‘waiting parents’ and ‘crying baby’.

But please remember that in the noun phrase, participles as adjectives do not behave like other adjectives. For example you can’t usually modify them with adverbs of degree: *the very waiting parents’ or *the slightly crying baby’, or make comparisons *’the most waiting parents’ or *‘the more crying baby’.

Regarding the position of adjectives, are you specifically referring to those adjectives that appear immediately after a noun (i.e. postpositive) such as 'anything useful', 'me included', or are you talking about adjectives appearing after the verb (i.e. predicatively) such as 'the girl felt unwell' or 'the man was loathe to leave'? If you say which, I may be able to help.

BillJ

* = ungrammatical