"I travel to the US every year."
Am I understanding right that this would be more like saying "I go traveling to the US every year" as a "word for word" translation rather than "I go to the US every year" or "I travel to the US every year"? Basically, we're saying we are "doing the traveling" like we "do the shopping"? (it sounds a bit weird...)
So you are saying that the literal translation says I go to the US for the sake of traveling? The English sentence Duo gives is "I travel to the US every year," and doesn't state what the reason is. It could be for the sake of traveling, it could be for the sake of business, it could be to see the Rose Bowl. I'm not seeing "for the sake of" in the sentence that Duo gives us. Is there another meaning for that phrase?
"For the sake of" isn't in the sentence because that's a literal translation. "I travel" implies enough that you are going there to travel, and if there was another reason it might be mentioned. I don't believe the word 旅行 is that specific in Japanese either, so I think "I travel" pretty much sums up what 旅行に行きます means.
Typically へ emphasises the movement verb and に emphasises the destination. If you were perhaps having a conversation about the fact you'll be away soon rather than where you will be, then へ might be more appropriate. e.g. "I won't be going to the party, every year around that time I travel to the US."
Both should be correct, although I also wonder if it's considered better style to use へ because you're stating the purpose of に行きます.
So it's the difference between; "I travel to the U.S. every year" being に vs "I travel (to the U.S.) every year." being へ?... Yeah, there is no clean way to really convey that in English over text. They really should either change the sentence to add the parentheses ( or possibly commas ) or they should accept に as a correct answer.
That would be why it's not に、but は can be used with times such as this, like "every year", or even "today", and "yesterday." The reason why there is no は is because 毎年 is not the topic of the sentence, and it is not being emphasized by は、but rather it's just a part of the sentence.
I was asked to translate "I travel to the US every year" and it did not accept "Maitoshi amerika he ikimasu". Why is the ryokou necessary in this translation? To me, travel is synonomous with "go", it's just describing moving from one place to another. Ryokou is holiday or vacation
I don't know which particle you used, so I'm just going to assume you used は？は marks the topic, so maybe you were already talking about "every year", or "every year" was the topic of your sentence. I don't see how that would ever be the case, as the literal translation would be "as for every year..." and you would pretty much never use it.