He had the opportunity to stand up - that is to say, it is as follows that [the parameters of the case of] the Mishna could be arranged: that he tarried, upon falling, enough time to be able to stand up, but did not stand up.

To shoulder - to correct [the position of] the beam on his shoulder, as those carrying a burden do.

And glaziers - sellers of glassware.

But if they all fell because of the first - this is explained later on. [Bava Kamma 31b:8]

Whichever way you [look at it] - since you have equated the property of the first person with a Pit, but [have declared damage caused by] the property of the second person to be exempt from liability.

If one who stumbles is considered negligent - and is considered as if he has discarded his nuisances in the public domain, and that is why [damage caused by] the property of the first person [causes him to incur] liability.

The second should also be liable - for damage rendered by his property to the body of the third person.

And if he is not considered negligent - and you are equating him with a victim of circumstances beyond his control.

Even the first should be exempt - even for damages caused by his own body, and how much more so for [damage caused by] his property.