As Mounce said, it's important to learn the articles of all the nouns because it's not always obvious what they should be from just the lexical form. A good example of that is ἡ ἔρημος (the desert, wilderness) which is a second declension noun. Most second declension nouns are masculine and have ὁ for the article, but ἡ ἔρημος is feminine and takes the feminine article.
It's also important to learn the stem of all the nouns because the lexical form isn't always a good indication of what it should be. For example, ἡ νύξ (the night) has for its stem νύκτ-, which isn't obvious. Another example is ὁ πατήρ, which might lead you to think that the stem would be πατ-, when actually it's πατρ-.
One good way to learn the stem is to learn the genitive singular and drop the ending. For example, the genitive singular of πατήρ is πατρός. By dropping the ending -ός, you get the stem πατρ-.