1) In the second stanza, the soldiers succeed in putting on their clumsy gas masks. Someone, however, is still crying out for help, unable to move quickly and easily as if stuck in fire or lime. [He couldn't get his own gas mask on fast enough.]
You can see how clumsy (awkward) these gas masks were and how they looked somewhat like helmets: http://www.google.com/images?q=wwi+gas+masks&oe=utf-8&rls=org.mozilla:en-US:official&client=firefox-a&um=1&ie=UTF-8&source=univ&sa=X&ei=R1uYTe-5II2btweZhPXvCw&ved=0CCMQsAQ&biw=1024&bih=413
2) Through the misty panes [the thick green-colored glass] of the gas masks and the thick green light, the poet saw his friend as if he were drowning in the deadly gas.
3) The memory of his dying friend returns in his dreams, and he cannot stop them. His friend moves towards him, guttering like a candle which is going to extinguish, choking and drowning.
In his dreams, the poet cannot help seeing his friend, lunging at him as he needed help and as he was dying.
4) He wonders if the readers can imagine walking behind a wagon into which their dying friends had been thrown once they couldn't walk.
5) He wonders if readers can see his friend's face, with its skin starting to fall off, if readers can hear the gargling sound as his friend tries to breathe with all the blood in his lungs.
5) In the first stanza, the British soldiers are bent over like old men, moving slowly towards their barracks. Many are walking along without their boots. [Remember that most of these guys are 18-22 years old.]
6)They hobbled along on their bloody feet, as if they were hurt or blind. They are so tired, they walk ask if they're drunk. They don't even seem to hear the sounds of the bombs and shells that were dropping behind them.
Dulce Et Decorum Est
Bent double, like old beggars under sacks,
Knock-kneed, coughing like hags, we cursed through sludge,
Till on the haunting flares we turned our backs
And towards our distant rest began to trudge.
Men marched asleep. Many had lost their boots
But limped on, blood-shod. All went lame; all blind;
Drunk with fatigue; deaf even to the hoots
Of disappointed shells that dropped behind.
GAS! Gas! Quick, boys!-- An ecstasy of fumbling,
Fitting the clumsy helmets just in time;
But someone still was yelling out and stumbling
And floundering like a man in fire or lime.--
Dim, through the misty panes and thick green light
As under a green sea, I saw him drowning.
In all my dreams, before my helpless sight,
He plunges at me, guttering, choking, drowning.
If in some smothering dreams you too could pace
Behind the wagon that we flung him in,
And watch the white eyes writhing in his face,
His hanging face, like a devil's sick of sin;
If you could hear, at every jolt, the blood
Come gargling from the froth-corrupted lungs,
Obscene as cancer, bitter as the cud
Of vile, incurable sores on innocent tongues,--
My friend, you would not tell with such high zest
To children ardent for some desperate glory,
The old Lie: Dulce et decorum est
Pro patria mori.
In #3 --> like a candle which is going to go out
In that Wikipedia link, you'll need to press Ctrl + F (for find) and type in 5.9 to locate the particular place where the term "five nines" is explained.
I should have double-checked verb tenses. For the most part, you should make sure the verbs in your sentences are in present tense. I'll put a corrected sentence below:
2) Through the misty panes [the thick green-colored glass] of the gas masks and the thick green light, the poet sees his friend as if he were drowning in the deadly gas.