(1) Spain is the country where I lived in when I was in my twenties.
→ (that / which) I lived in / where I lived / in which I lived
(2) What do you say to going to the restaurant Jim tould us?
→ the restaurant (that / which) Jim told us about
(3) Speaking a language is one of the most imporatnt ways how humans communicate with each other.
→ howを削除 / in which / that
(4) I cannot believe what William said such a cruel thing to you. → that
(5) New York City where there are a variety of ethnic groups is very exciting.
→ City, whereとgroups, isとする。（Cityとwhereの間、およびgroupsとisの間にコンマを入れる）
（解答） Paris is one of the most beautiful cities (that) I have (ever) visited.
（別解） Paris is among the most beautiful cities (that) I have (ever) been to.
（解答） According to recent research, those [people] who jog [go jogging] for about thirty minutes [for half an hour] several [a few / a couple of] times a week will probably [are very likely to] live longer (than those who do not) [have a long life / live (for) a long time / live a long life].
（別解） A recent study shows that if you spend about thirty minutes jogging several days a week, you will probably live a long life.
（解答） The time [The days / The age / The era] will soon come when robots (will) do a lot of work [a lot of jobs / many jobs / a lot] for humans [for human beings / instead of humans / instead of human beings].
（別解） It will not be long before robots work a lot for humans.
（解答） Children are always [always look] very [really] happy [glad / delighted] to discover or learn [get to know] (new) things (that / which) they do not know [did not know before].
（別解） Children are very happy when they make new discoveries or learn new things.
（解答） I have been thinking for a long time about [of] taking you out to eat [taking you out for] a typical [traditional] Japanese meal (that / which) you probably have never eaten before.
（別解） I have always wanted to take you to a Japanese-style restaurant and buy you food (that / which) you probably have not tried yet.
（解答） In ancient times, a large part of Japan, which has [gets] a lot of rain, must have been covered with [by] forests [forest].
（別解） Japan has a lot of rain, so it must have been mostly covered with forest in ancient times.
（解答） Yokohama, which used to be known as a gateway to the world, (is the city that) has the second largest population in Japan now.
（別解） The population of Yokohama, (which was) formerly known as a gateway to the world, is now the second largest in Japan.
（解答） My father, who used to work [once worked] for a foreign affiliated company, is always telling [always tells] me learn another language in addition to [besides / as well as / on top of] English.
（別解） My father, having worked for a foreign affiliated company, is always teling me that I should learn another language, not only English.
（解答） If you want to [would like to / are to] get along (well) with [get on well with / be on good terms with / become good friends with] other people, you should avoid saying something that [things that] could [would / might] make them angry.
（別解） In order to get along with other people, be careful not to say something that would make them angry.
（解答） A girl who used to prefer reading (books) alone to playing with other children is now playing an active [important] role on [in] Japan's national football team.
（別解） A girl who used to prefer to read (books) alone rather than to play with other children is now active as a member of Japan's national football team.