Learn Russian: Он никогда не забывает свой пароль.

In Russian, свой is a reflexive possessive pronoun that refers back to the subject of the sentence. Since "he" (the subject) is remembering "his own" password, we must use свой. If we used его, it would mean he never forgets someone else's password (like another man's password).

Why is свой in this exact form, and not своего or свою?

Свой agrees in gender, number, and case with the noun it describes. The word пароль is the direct object receiving the action of forgetting, so it is in the Accusative case. Because пароль is a masculine, inanimate (non-living) noun, its Accusative form looks exactly like its Nominative (dictionary) form. Therefore, the pronoun also takes the inanimate masculine Accusative form: свой.

The word пароль ends in a soft sign (-ь). How do we know if it is masculine or feminine?

Nouns ending in a soft sign can be either masculine or feminine, and you often just have to memorize their gender. However, in this sentence, the pronoun gives it away! Because we use the masculine form свой instead of the feminine form свою, we know that пароль is masculine.

Why does the sentence use both никогда (never) and не (not)?