Good website, but I have a question about this adjective (maybe all adjectives?). The chart says that the basic (positiv) adjective is groß, but I don't see any occasion when that form of the word would be used. Even the nominative masculine form gets inflected to großer. Is there any German sentence in which the "basic" form (groß) would be used, without inflection?
"Predicate adjectives, i.e. adjectives that don't precede a noun, are not inflected.
Der Mann ist groß.
Die Männer sind groß.
Die Frau ist groß.
Die Frauen sind groß.
Das Haus ist groß.
Die Häuser sind groß.
As you can see, the adjective remains in the base form, regardless of number and gender."
@Villy84 : The gender (der Hund) of the dog (Hund) needs to be shown. Since the -er is not attached to the ein, it gets attached to the groß = großer. This would be the simple explanation. For grammar rules see http://en.wikipedia.org/wiki/German_adjectives
By the way, in this sentence mixed inflection is used due to the indefinite article ein.
The verb describes the dog, which is the subject of the sentence. "Der Hund" is in the nominative case. The dative "dem" is the article for the window, not the dog. The window is where the dog is located, so "das Fenster" becomes "dem Fenster."
Another example: I would say "Mein Buch ist auf dem Tisch," not "Mein Buch ist auf der Tisch."
The adjective is preceded by ein, so it has to be mixed inflection. (der) Hund is masculine, and ein and ist tell us that it's singular and nominative. The mixed inflection ending for masculine singular nominative is -er.
Wikipedia has a good set of tables about adjectival endings and the different (weak/mixed/strong) inflections - here's the mixed inflection one: http://en.wikipedia.org/wiki/German_declension#Mixed_inflection