Adjectives, including the article, agree with the nouns that they modify, in gender, number, and case.
Examples:
1. ὁ λόγος, τοῦ λόγου, τῷ λόγῷ, βλέπω τὸν λόγον, οἱ λόγοι, τῶν λόγων, τοῖς λόγοις, βλέπω τοὺς λόγους.
2. τό δῶρον, τοῦ δώρου, etc.
3. ἡ ὥρα, τῆς ὥρας, τῇ ὥρᾳ, βλέπω τὴν ὥραν, αἱ ὥραι, etc.
4. ἡ ὁδός, τῆς ὁδοῦ, τῇ ὁδῷ, βλέπω τὴν ὁδόν, αἱ ὁδοί, τῶν ὁδῶν, ταῖς ὁδοῖς, βλέπω τὰς ὁδούς.
5. καλὸς λόγος etc., καλὴ ὥρα, καλὴ ὁδός etc.