"They will not go to the restaurant."
Translation:Вони не будуть іти у ресторан.
I notice that both їхати and йти end in the same vowel, but the two correct answers do not both use the same "в" or "у." Please explain why it's "їхати У ресторан" but "йти В ресторан."
All versions are accepted. Currently, the sentence that is displayed to learners in Ukrainian (when translating to English) can be: Вони не будуть [іти/йти/їхати] [у/в] ресторан. So, any combination can be shown randomly. It could be that when you got this exercise several times, it happened to be "їхати у" and "йти в". I personally say "в", but it is not a rule (as nothing in a language on the street..).
In this course it's agreed not to be strict about the usage of у/в because: 1) even the native speakers are flexible about it depending on the region 2) it's too strict for a fun course on the Internet :)
When "у/в" indicates a direction of movement, the next noun is used in accusative case: "зайти в будинок", "покласти в коробку", "стрибнути у воду". The locative is used when "у/в" means the location: "я сиджу в кімнаті", "пальто вісить у шафі", "ми обідаємо в ресторані". So, with the locative case this sentence would mean something like "they will not be walking inside the restaurant".
Both are correct and both are accepted.
Note: their usage is slightly different in Ukrainian, not 100% equivalent.
У ресторані = inside the restaurant
ресторані is in the singular, locative case.