参考翻译及详细解释：
1. The weather is fine and we want to visit the sea park which/ that was built last week.
汉语用的是“建成”，所以可以用be built,或者be completed,前者更好。关系代词两个都可以。“天气好”的表达比较多，还可以翻译为：It is sunny, the weather is nice/lovely等。
2. He sent me a book, for which I gave him a dictionary in return.
for which即”为此、因为这本书“，而in return作为回报。由于关系代词在介词后边，只能用which，不可用that。因为动作已经发生，时态用一般过去时。
3. Are you coming to see a movie/ film?
come,go,arrive等表示位移的动词经常用现在进行时表将来。”电影“美式英语用movie，英式英语用film。看电影除了上面的两个说法外也可以翻译成：go/come to the cinema/movies.
4. He was laughed at by his classmates because of his foolish mistakes.
后半句也可以翻译成：because he had made some foolish mistakes.因为“犯错误”在先，“被嘲笑”在后，所以“犯错误”应该用过去完成时。because of 是介词词组，后面加名词或代词；because是连词，后面接一个句子。
5. From then on, both his English speaking and writing improved rapidly with the help of the /my teacher.
注意from then on后边时态用一般过去时。此外，"英语口语"也可以说成oral English。”老师“在此处为特指，可以用定冠词the或形容词性物主代词my。
6. My friend Jack is English and he is good to me, for which my parents often invite him to my home for dinner.
或者翻译成：
My friend Jack, who is an Englishman, is good to me and my parents often invite him to my home for dinner.
”某某是英国人“有两种说法，可以用English或Englishman/Englishwoman，注意前者是形容词，后者是名词，应当注意冠词、性别。 Tom is English. =Tom is an Englishman. Mary is Engish. = Mary is an Englishwoman.
第一种翻译用非限定性定语从句，which代替前面整个句子，意思即”出于这个原因“；第二种翻译也用了非限定性定语从句，插入句子中间作为补充说明。
希望对你有所帮助:-) 祝学习进步！
