1. Sadly, it is very difficult for giant pandas to survive in the wild. → survive=continue to live
〈知识链接〉sadly置于句首时表示对某件事情感到惋惜、难过。类似的有luckily, unluckily, hopefully。
It is+形容词+for sb to do sth. It是形式主语，而动词不定式是真正主语。
It is necessary for Oxform Trailwalkers to support each other during the charity walk.
2. If a bear is in danger, it will attack people. 如果熊处于危险中，它就袭击人。
3. Tigers will live as a family if they have babies.如果老虎生了虎崽，就生活在一个家庭里。
4. Tigers will live as a family until baby tigers are 2-3 years old.
⑴She slept till/until nine.=She stopped sleeping at nine.她一直睡到九点钟。
⑵She didn’t go to sleep till/until nine.=She started sleeping at nine.她九点钟才开始睡觉。
They talked until dawn.
They did not return home until 10:00.
③I didn’t know it until you told me. His uncle didn’t get married until he was 35.
④—Was his father very strict with him?
—Yes. He had never praised him until he became one of the top students in his grade.
They the train until it disappeared in the distance.
A. saw B. watched C. noticed D. looked at
〈知识链接〉当until从句与肯定的主句连用时，主句的谓语动词必须是延续性动词;与否定的主句连用时，主句的谓语动词必须是非延续性动词。Don’t get off until it has stopped.