Adjectives: (lack of) agreement with their nouns

In the context of grammar, "agreement" refers to the matching of words in number and gender.  Most of the time, an adjective does not "agree" with the noun it is modifying (but of course there are some exceptions).

Perhaps Wikipedia states it best when it writes that:

Agreement [can occur] between nouns and their modifiers, in some [languages]. This is common in languages such as French, where articles, determiners and adjectives (both attributive and predicative) agree in number with the nouns they qualify:

• le grand homme ("the great man") vs. les grands hommes ("the great men")

In English this is not such a common feature, although there are certain determiners that occur specifically with singular or plural nouns only:

• One big car vs. Two big cars
• Much great work vs. Many great works

So, for example, here are some correct and incorrect ways of writing sentences, based on real homework that I have received from students:

Correct	Incorrect
I had many traumatic memories as a child.  Today, those memories follow me everywhere.	I had many traumatic memories as a child.  Today, thoses memories follow me everywhere.
Fortunately, there are not just negative things in my life.	Fortunately, there are not just negatives things in my life.

eHow explains the exceptions, as follows:

[Where appropriate, an] adjective must always "agree" with its noun or pronoun by matching its number. Singular nouns take singular adjectives, while plural nouns take plural adjectives. For example, it would make no sense to say "two book," because "book" is singular while "two" is plural. We instead say "two books." Similarly, the phrase "a blue trees" lacks agreement because "a" indicates just one tree, while "trees" is plural. We must therefore change the phrase either to "a blue tree" or to "the blue trees."

Help!  I cannot find any decent websites that discuss this topic in detail.  If you discover any, please let me know.