In German, there are two basic types of adjectives:
I.) predicate adjectives, i.e. adjectives used after the verbs "sein" (to be) and "werden" (to become). They don't get an ending.
Ex. Das Tier ist klein. (The animal is small.)
II.) attributive adjectives, i.e. adjectives used before a noun (e.g. a small animal, my green shirt, the nice man ...). They get an ending.
The ending depends on three factors:
1.) the type of article word used with the noun the adjective refers to (e.g. the, a, my, no article ...)
2.) whether the noun is masculine, feminine, neuter OR plural
3.) the case of the noun
E.g. here, the noun the adjective "small" refers to is "animal" (German: Tier).
1.) "Tier" is used with the indefinite article "ein" ("a") (= so-called mixed inflection)
2.) "Tier" is neuter
3.) "Tier" is accusative (it's the direct object in this sentence).
The attributive adjective ending for mixed inflection, neuter, accusative is -es: klein-es
das kleine Tier, ein kleines Tier = noun as (direct) object in accusative case, the adjective has to agree: http://german.about.com/library/weekly/aa033098.htm
The reason is that some verbs in English (called stative or non-continuous verbs) are not used in continuous tenses. The verb "to see" is one of these stative verbs. So you can say "he sees ...." but not "he is seeing...". Of course many people probably do say "he is seeing ..." but nevertheless that is the reason why it is grammatically suspect/wrong depending on context. http://www.perfect-english-grammar.com/stative-verbs.html