Rules for adjective endings:

There are three zones in the noun phrase. 

The first zone is the determiner zone.  It contains at most one item.  That item must be a determiner.  A determiner is a special sort of adjective with a grammatical function, like the, a, some, every, and so on.  The set of determiners in German can be further broken down into the der- words, like: der, die, das 'the'; dieser, 'this'; jeder, mancher, 'some' etc. and the  ein-words, like: ein 'a', kein, 'no', viel 'much', and the possessive adjectives mein, dein, sein, ihr, unser, euer).  Ein-words do not take endings in the masculine nominative singular, and the neuter nominative/accusative singular (the shaded cells in the chart below).

If you can put a strong ending in the first zone, then you must do so, and all following words will have weak endings.

The second zone is the attributive adjective zone.  This zone contains indefinately many items (from zero on up).  Attributive adjectives are descriptive words, and it is usually easier to provide a definition for an attributive adjective than for a determiner (i.e. it is easier to say what loyal means than what the means).  Attributive adjectives are words like yellow, green, happy, sad, furious, ugly, funny, childish, friendly, etc.  Grammatically, they can go between the and the noun (the ugly little yellow childish duckling).

If there is a strong ending in the first zone, then all items in the second zone have the appropriate weak ending.  If there is no strong ending in the first zone (either because the determiner is ein or because there is no deteriminter), then all items in the second zone get strong suffixes.

The third zone contains the noun.  Typically there is only one noun per noun phrase.  Usually German nouns don't take the strong or weak endings, but they may undergo some change in the plural (adding a suffix, changing a vowel, or some such) this doesn't have anything to do with the endings on the modifiers (determiner and attributive adjectives).  The non-feminine Genitive singular is an exception, but we are ommiting the genitive from consideration, right now.)

Starke Endungen (Just one, as far left as possible)

Schwache Endungen (attributive adjectives only, only if there is a strong ending to the left, all adjectives are identically inflected)

Here are some examples.  Use the rules and tables above to figure out what is going on. The shaded cells contain weak endings.