"To be excited/elated" Something around those lines.

"して貰う" is usually used in order to say "I had something done for me"

女の子は彼氏に荷物を運んでもらった”＝The girl had her boyfriend carry [her] luggage.

The "A" in the sentence is the one performing the action.

 

無 is the same as ない. So it means "nothing" or "doesn't exist" 敵 means "rival" or "opponent" So I think the nuance may be a little different than the English word, but you can probably use it interchangeably in a way that can be understood.

アメリカは日本と逆で、クリスマスは大体の人が実家に帰って家族と一緒に祝う祝日です。忘年会という文化がないですけれども、New Year's Eveっていう日はそんな感じだろうと思っています。 日本での忘年会は経験したことがありませんが、アメリカでの大晦日は友達と一緒に飲んだり遊んだりしますね。

I think 「ひとつ」 is used as a way to express that there are a number of concerns for the Japanese people, but the aging population is one of those concerns. So, I think either sentence is grammatically correct, it just depends on whether you want to convey the message of a broader spectrum（「一つ」を使う場合）Or a more specific sentence which only introduces the single topic of the aging population.

Kanji is a style of writing borrowed and modified from the Chinese language. For most Kanji there are two readings: "On"(taken from the original Chinese pronunciation) and "Kun"(Modified form original to Japanese).

"Are your things packed? Are you moving far away?"