"Bye, good night!"
Translation:Ahoj, dobrou noc!
Is ahoj is similar to čau? Cuz I thought that ahoj is only used as informal hello.
Yes, Ahoj is more or less on the same level as Čau and I use one or the other quite randomly.
Is this in accusative case? (dobrá -> dobrou) I feel it's somewhat unfair for the course to expect me to know because the basics have only been teaching me nominative so far.
You are right, it's accusative. Yes it's a bit unfair but phrases are a very special category. They are mostly the first thing you learn in any language even if you don't know the grammar of that language yet.
The accusative case (abbreviated acc) of a noun is the grammatical case used to mark the direct object of a transitive verb. The same case is used in many languages for the objects of (some or all) prepositions. It is a noun that is having something done to it, usually used together (such as in Latin) with the nominative case. For example, "they" in English is nominative; "them" is accusative. The sentence "They like them" shows the nominative case and accusative case working in conjunction using the same base word. The syntactic functions of the accusative consist of designating the immediate object of an action, the intended result, the goal of a motion, and the extent of an action.[1
Could you explain what the Accusative form means? I understand there are 7 cases, but how do you explain its purpose or meaning?
The accusative form is used, because it's short for "I (nominative) wish you a good night (accusative)". Actually, dobrý den/dobré ráno/dobrý večer are also accusatives. It' s just not as obvious as for feminine nouns, as the nominative and accusative forms are identical for neuters and inanimate masculines.
Yes, it really is in accusative, you are right. But don't worry about the cases now, you can learn all about them later in the course, right now focus on the simplest phrases and greetings :)