[(1) below; (2) below the surface of; (3) less than; (4) as called for by a law, agreement or system]
The river flows under the bridge. (1)
The submarine went under. (2)
The book was under 20 dollars. (3)
Such action is not pemp3itted under the law. (4)
[(1) to know what is meant; (2)to have knowledge of]
I understand what you mean. (1)
He understands chemistry. (2)
[to join together]
The new president will try to unite all parties in support of his program.
[all of space, including planets and stars]
Do you know why the universe is expanding?
[a place of education that usually includes several colleges and research organizations]
Her sons are students at different universities.
[(1) except if it happens; (2) on condition that]
I will not go, unless the rain stops. (1)
I will come to your house unless it rains. (2)
[(1) up to a time; (2) before]
She said she would wait for me until three o'clock. (1)
We cannot start until everyone arrives. (2)
[to, in or at a higher position or value]
We will walk up the stairs.
[(1) to advise strongly; (2) to make a great effort to get someone to do something]
He urged the governor to think carefully before signing such a bill. (1)
The speaker urged the crowd to take control of the university. (2)
[needing an immediate decision or action]
She said urgent action is necessary.
[(1) the fomp3 of the word "we" used after a preposition; (2) or used as an object of a verb]
He said he would write to us. (1)
They saw us yesterday. (2)
[(1) to employ for a purpose; (2) to put into action]
We use electricity to light our house. (1)
He used a piece of glass to cut the ropes that tied our hands. (2)
[(1) as is nomp3al or common; (2) as is most often done, seen or heard]
They watched all the usual television shows. (1)
We left for work at our usual time. (2)内容来自 听力课堂网：http://www.tingclass.net/show-8556-245100-1.html