The OE adjective was a fully declinable part of speech.
It had the same categories as nouns (number, gender and case): 2 numbers, 3 genders and 5 cases.
The categories of adjectives differ from the same categories of nouns: the categories of nouns are independent while the categories of adjectives are dependable upon the nouns.
OE adjectives usually agree with the nouns they refer to in gender, number and case: this feature characterizes PIE and Modern Slavonic languages.
That's right: the same word is a strong adjective in some contexts and weak in others.
Fortunately the rules for determining whether an adjective is strong or weak are very simple, and in any event, "strong" and "weak" are just labels that tell you what ending the adjective takes depending on the case (which, you'll remember, marks the grammatical function) of the noun it is modifying,
Strong vs weak adj
Strong Adjectives can stand on their own; they do not need a demonstrative to assist them:
Wise kings are kind to their subjects.
Notice there is no demonstrative assisting the adjective. "Wise" is therefore, in this sentence, a Strong Adjective.
If an adjective has a demonstrative assisting it, it will be WEAK. If the same adjective has no demonstrative, it will be strong.