Your notes state it flatly: in Spanish the full stop and the comma change places.

4,90 € is four euros ninety, read as cuatro euros con noventa centimos.

1.500 € is one thousand five hundred euros.

The trap: writing 4.90 in a Spanish answer reads as four euros and a bit of nothing. And the euro sign goes after the number.

costar is o to ue and is used almost only in the third person: cuesta for one thing, cuestan for several.

¿Cuanto cuesta un bocadillo? ¿Cuanto cuestan los bocadillos?

The trap: your class table gives a full paradigm for costar with forms like cuesto and costamos. Ignore them. Things cost; people do not.

el agua takes el because la agua would be hard to say, but the noun is feminine: el agua fria, las aguas.

Same for el aula, the classroom.

The trap: el agua frio is wrong. The article lies, the adjective does not.

¿Que tomas? is the normal way to ask what someone is having, food or drink.

Then the modifiers: (muy) caliente, (muy) frio, del tiempo (at room temperature), con leche, con hielo, con limon, con o sin gas, con o sin azucar.

The trap: del tiempo has nothing to do with the weather here. It means not chilled.