I'll give you some ideas, but I'm not going to write something for you.
Well, Trench warfare was just horrible. After it would rain (and then remain humid) in the battlefield, not only would the ground they were standing in become mud, the bottoms would fill with water. So they would be standing for hours, days, or even weeks in water that was seeping through their clothes and boots, and it would get so bad that the skin would just peel off of their feet (look up pictures of trench foot).
There was also the concept of gas warfare which was used for the first time. Wikipedia would provide you with good examples of gasses used in the first world war, but I wouldn't cite it if I were you :P
Not to mention the strange combination of boredom and terror as you would have to stay in a trench for months on end, fearing enemy machine gunners, artillery strikes, gas attacks (later in the war), and snipers that would take your head off if you so much as peeked your head over the trench wall.
I'm not quite sure what your teacher means by "the reality" though, so I would just use a lot of descriptive words if I were you.
