"The woman drinks a cup of coffee."
Translation:Kvinnan dricker en kopp kaffe.
There is a rule, a bit tricky but works with some exceptions. In your case, it's "en kopp" (a cup). The cup, is "koppen". Let's try this but with "ett". "Ett tåg" (a train). The train, is "tåget". So here is the explanation: If you put "the" in front of the word in english, and translate it to swedish, the word will end with either -en or -et. If the swedish word ends with -en, you put "en" in front of it if you mean a specific thing. If the swedish word ends with -et, you put "ett" in front of it if you mean a specific thing. This is just for singular though.
En and ett are grammatical genders: like in German (der, die, and das) or French (le and la).
Except that in Swedish, masculine and feminine words have a long time ago merged into the common gender (en-words), and there are a lot more of these than of the neuter gender (ett) words.
You just have to learn the gender along with the word, so "en kopp" and "ett tåg". If you have no clue with a particular word, go for en, as they are much more numerous.