Question

It isn't easy for the students across the river 中

It isn't easy for the students across the river 中为什么要用across

Answer 1

这里表示横穿河流所以用across

另外，附上across具体用法：
1. 无论用作介词还是用作副词，均可表示动态意义或静态意义：　　
(1) 表示动态意义，意为“横过”“到…的另一边”。如：　　
Can you swim across? 你能游过去吗? （副词用法）　　
I cam swim across the river in 10 minutes. 我可以在10分钟之内游过河去。（介词用法）　　
(2) 表示静态意义，意为“在…的另一边”。如：　　
My house is just across the street. 我家就在街道对面。（介词用法）　　
We leave Dover at ten and we should be across in France by midnight. 我们10点钟离开多佛，午夜时应能抵达法国那边。（副词用法）　　
2. across from与from across都可以说，但含义稍有区别：　　
(1) across from 表静态意义，意为“在对面”，与opposite同义，此时也可省略from只说across。如：　　
Just across from our house there’s a school. 就在我们房子的对面有一所学校。　　
I’m staying at a little hotel just across from the Libyan Embassy. 我住在利比亚大使馆正对面的一所小旅馆里。　　
(2) from across 则表动态意义，意为“从……的对面”。如：　　
He shouted to me from across the room. 他从房间的另一边向我喊。　　
They waved at us from across the room. 他们从房间那一头向我们招手。　　
3. 注意不要与动词cross弄混(www.yygrammar.com)：　　
(1) 表示“穿过”时的用法比较：　　
It’s dangerous for an old man to cross such a crowded street. 老人穿过这样拥挤的街道很危险。　　
It’s dangerous for an old man to walk across such a crowded street. 老人穿过这样拥挤的街道很危险。　　
(2) 表示“交叉”时的用法比较：　　
The roads cross just outside the village. 那两条路就在村外交叉。　　
He sat there with his arms across his chest. 他坐在那儿，两臂交叉放在胸前。（介词用法）
4. 用于习语come across，主要用法有：　　
(1) 偶然遇见，碰见（=run across）　　
Where did you come [run] across her? 你在那儿碰到她的?　　
I’ve just come across a beautiful poem in this book. 我在这本书里偶然发现一首优美的诗。　　
(2) 被理解，被传达（=come over）　　
He spoke for a long time but his meaning did not really come across. 他讲了很长时间，但他的意思没有人真正理解。　　
(3) 使人产生某种印象（=come over）　　
Your speech came across very well. 你的演说相当受欢迎。　　
He came across as sympathetic / a sympathetic person. 他给人以有同情心的印象。　　
注意以下用法，它不是习语，其中的across为副词，表示“在另一边”：　　
Come across to my office this afternoon. 你今天下午到我办公室来一趟。

Answer 2

across 是从物体上方横穿过去

Answer 3

应是：It isn't easy for the students to cross the river