I did the corrections myself. I included them in brackets. Can you see if they are correct? Thank you.

1)The subject of the poem is the description of some soldiers while they are retreating through (toward) their trenches.
2)They should be young and strong but, because of their tireness (tiredness) the poet describes them as old beggars.
3)The soldiers are described as moving slowly through the mud. They have no boots, so their feet blood (bleed). Their march is slow and uneasy: that’s why (That’s why) they walk as if they are blind or drunk or stuck in mud and water. (blind, drunk or stuck in water).
4)In the second stanza the poet writes about how few soldiers manage to wear their clumsy gas masks. Anyway (?), some of them are unable to move easily and fast enough; for this reason (For this reason,) they still cry out (they are still crying out) in the deadly gas.
5)The third stanza deals with the memory of a friend of the poet’s ; he (The poet) can’t help seeing him in his dreams, guttering like a candle which is going to extinguish.
6)Finally, in the last stanza, the poet wonders if the reader can imagine how it feels like walking behind the wagon when they throw his friend (where they had thrown his friend), or watching his eyes (his friend’s eyes) writhing and the skin of his face fall off.

3. bled

4. As a result; for this reason; still cry out is OK
6. off, his friend's eyes.