句型翻译
I've learnt Japanese in the past.
我学过日语。
I've eaten Thai food before.
我吃过泰国饭。
I've never seen his girlfriend before.
我没见过他的女朋友。
I've never used chopsticks before.
我没用过筷子。
I've never bought a plane ticket before.
我从没买过机票。 （Without it, the sentence could possibly mean you haven't bought the ticket you need now.）
Previously I didn't enjoy eating beancurd.
以前，我不喜欢吃豆腐。
Previously I had never learnt Mandarin.
以前，我没学过普通话。
Previously I couldn't use chopsticks.
以前，我不会用筷子。
At some time in the future I plan to go to Thailand to look for my girlfriend.
以后，我打算去泰国找我女朋友。 （With the character, it means you already have a girlfriend and would like to meet her in Thailand. Without it, the sentence means you don't have one and would like to find one there.)
At some time in the future I must learn Korean.
以后，我必须学韩语。
At some time in the future I can teach English.
以后，我可以教英语。
At some time in the future I will rest for two weeks.
我打算休息两个星期。 （The original sentence doesn't make the sense you want, though it is grammatical.)
Next year he is able to teach for a year.
他明年可以教一年书。 （You need an object here to be grammatical. But this sentence doesn't mean the same thing as your English sentence. It actually means he's able to teach for a whole year next year. You English sentence, however, probably means he can teach from a certain point of time continually for a year. You actually need two Chinese sentences for the original English sentence.)
This year I will study for two weeks.
今年，我会学习两个星期知识。
Today the child must write for half-an-hour.
今天，孩子必须写半个小时作业。
The plane is still going to fly for 20 minutes.
飞机还要飞20分钟。
How long are you thinking of playing for?
你想玩多长时间？
Next year he is able to teach English for a year.
他明年可以教一年的英语。
This year I will study Thai for two weeks.
我今年会学两个星期的泰语。
Today the child must write Chinese characters for half-an-hour.
孩子今天必须写半个小时的汉字。
How many days do they plan to walk for?
他们打算走几天？
句型翻译
I've learnt Japanese in the past.
我学过日语。
I've eaten Thai food before.
我吃过泰国饭。
I've never seen his girlfriend before.
我没见过他的女朋友。
I've never used chopsticks before.
我没用过筷子。
I've never bought a plane ticket before.
我没买过机票。
Previously I didn't enjoy eating beancurd.
我以前不喜欢吃豆腐。 or 以前，我不喜歡吃豆腐。
Previously I had never learnt Mandarin.
我以前没学过普通话。 or 以前，我沒學過普通話。
Previously I couldn't use chopsticks.
我以前不会用筷子。 or 以前，我不會用筷子。 or 我曾經不會用筷子。
At some time in the future I plan to go to Thailand to look for my girlfriend.
我打算以后去泰国找我女朋友。
At some time in the future I must learn Korean.
以后，我必须学韩语。 or 我以後必須學韓語。
At some time in the future I can teach English.
以后，我可以教英语。or 我以後可以教英語。
At some time in the future I will rest for two weeks.
以后我会休息两个星期。 say 我將會休息兩個星期。
(unnatural)
Next year he is able to teach for a year.
他明年可以教一年。 better say 他明年可以教一年XXX。 or 他明年可以任教一年。 or 他明年可以做一年教師。
（"教" is considered to be a transitive verb, although your sentence is sometimes used.）
This year I will study for two weeks.
今年我会学习两个星期。 or 我今年會學習兩個星期。 or 今年，我會學習兩個星期。
Today the child must write for half-an-hour.
今天孩子必须写半个小时。 better say 今天，孩子必須 寫作/抄寫/書寫 半（個）小時。
（"寫" is also considered to be a transitive verb.)
The plane is still going to fly for 20 minutes.
飞机还要飞（行）20分钟。
How long are you thinking of playing for?
你想玩多长时间？
Next year he is able to teach English for a year.
他明年可以教一年的英语。
This year I will study Thai for two weeks.
我今年会学两个星期的泰语。
Today the child must write Chinese characters for half-an-hour.
孩子今天必须写半个小时的汉字。
How many days do they plan to walk for?
他们打算走几天？
