"Andrew is writing with the pen on the red notebook."
Translation:Andrei scrie cu pixul pe caietul roșu.
In general, nouns introduced by prepositions other than "cu" will lose the definite article that you normally use in English. Notable exceptions to this ~rule are the cases where the noun is qualified by an adjective, a possessive pronoun, a "postfix" demonstrative pronoun, etc.
In the above sentence, we have the adjective "roșu" modifying the noun, so this causes the definite article to be used and we get "caietul".
Here are some other examples, to make it clear:
- The notebook is on the table. = Caietul este pe masă.
- The notebook is on the red table. = Caietul este pe masa roșie.
- The notebook is on my table. = Caietul este pe masa mea.
- The notebook is on Andrew's table. = Caietul este pe masa lui Andrei.
- The notebook is on that table. = Caietul este pe masa aceea.
fountain pen = stilou
ball pen = pix
It's kinda tricky to translate "pen" into Romanian, since you generally need to know which kind of pen it is... and since ball pens are much more common than fountain pens, it's usually translated as pix, but I suppose stilou is passable too.