German Grammar Roadmap (link)
Changing articles with cases:
Source: The Four German Cases (link)
Accusative object is the direct object in a sentence. Example: "Ich sehe mich."
See Accusative Pronouns (link) at Duolingo.
Possessive case. See Wikipedia.
The dative object is an indirect object in a sentence. Example: Ich gebe das Buch meinem Vater. Ich gebe dir ein Buch.
Also, when used with mit, bis, zu, neben, etc. "Wir spielen mit der Katze."
'In' is a preposition which may go with either dative or accusative case. Depending on the gender of the noun that results in 'in dem', 'in den', 'in die' or 'in das'. 'Im' is a contraction of 'in dem', so that it only applies to masculine words in dative case. (Source: see here)
A pronoun is a word that represents a noun, like "er" does for "der Mann." In the nominative case, the personal pronouns are simply the grammatical persons you already know: "ich," "du," "er/sie/es," "wir," "ihr," "sie," and "Sie."
The demonstrative pronouns in English are: this, that, these, and those. In German, the demonstrative pronouns in the nominative case are the same as the definite articles. That means, "der," "die" and "das" can also mean "that (one)" or "this (one)" depending on the gender of the respective noun, and "die" can mean "these" or "those." For example, if you talk about a certain dog, you could say "Der ist schwarz" (that one is black).