In the last line of the first stanza, the speaker refers to the “gas shells dropping softly behind” the soldiers as they trudge through the mud of the battlefields. The first line of the second stanza begins with someone shouting, “Gas! Gas! Quick, boys!” instructing the soldiers to get their gas masks on fast so that the poison gas does not enter their lungs and kill them. However, one of the young men does not manage to get his mask on in time, and he is described as “yelling out and stumbling” around, all the while breathing in the poison gas.
The speaker uses a series of similes to describe the man’s appearance. First, the speaker says that the now-doomed soldier seemed to be “flound’ring like a man in fire or lime.” If you imagine a person who is engulfed in flame or covered with some caustic, burning substance, it is not hard to also imagine how that man might flail his arms, clutch at his exposed skin, and so on. Second, the speaker says, “As under a green sea, I saw him drowning,” another simile to describe the poor soldier’s appearance and eventual fate. The “misty panes” are the windows in the gas mask that the wearer looks through, and it could be that those little windows turn everything green because of their own green hue, or it could be that the mist and haze of the gas emitted from the shell is actually green in color. Either way, the soldier is not actually drowning but, rather, appears to be drowning because of the way he physically responds to inhaling the poison gas that will eventually kill him.