Normally you use always accusative. Here the genitive you proposed would be clearly wrong.
You use genitive when coupled with words of quantity like "trochu", "mnoho", "hodně", "málo", where you often use "of" in English.
Then sometimes you can use the partitive genitive, especially in negative sentences, but that is an andvanced topic. The above mention cases actually belong into this category, but there is more where you can sometimes use genitive, but it may sound archaic and it is really advanced. For example "Namám kávy." is theoretically posible, but will sound archaic or just strange. Just use accusative.
does the adjective always follows the noun? for exemple why not saying donrá kávu instead of dobrou kávu?
This is the difference between the object and subject of the sentence. You should really study the Tips and notes. You can read other resources about grammatical cases.
Due to various processes in history certain parrts of English grammar became extremely simplified and are different from other Indo-Eropean languages. Do not take English as a typical example, it is very atypical.
Fascinating. Thanks so much. I did look at the tips and notes and at the table but didn't find much of an explanation what the "accusative" was nor its difference from "nominative". I just got Naughton's book on Czech grammar this morning that has been referenced several times in these discussions as well as a better dictionary. I'll dig around in those.
From the tips for Food:
Please read the introductory paragraphs on cases and the summary of the adjective endings in the nominative and the accusative in the neighboring Animals skill. We needed to conserve room here.
And from the tips for Animals:
Above this row of the tree, we were dealing almost exclusively with the verb "be" and with nouns and adjectives in the nominative case. That would only take us so far. Maybe we could talk about what or who something or someone is, what something or someone is like, or (to some extent) where something or someone is. But if we are ever going to move from states to actions, we will need more verbs and more cases.
Simply put, the case is a grammatical category that provides information on the function of the word (usually a noun, adjective, pronoun, or numeral) relative to the other words around it. In English, much of this information comes from the position of the word. Czech is one of the languages with a fairly free word order, and other clues are needed.
The nominative case is used to "name" the subject of a verb, i.e., the "doer" of whatever action is being described. When we say František je vysoký. (František is tall), "František" is in the nominative case. (So is "vysoký".) If František eats something instead of just being tall, he will still be in the nominative, but what he eats will be in a different case.
The accusative case is mostly used to mark the object of a verb, i.e., the target of the action, and often without preposition. Whatever František is eating normally ends up in the accusative.
To tell the accusative from the nominative, we need to pay attention to the endings, just like we did when making the plural.