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Context:

"I'm only 49 years old and this isn't supposed to happen to me," Henry says.

It's not supposed to be this hard to walk either.

"My knee is weak, but I have what they call foot drop," Henry explains.

Small steps are a struggle that leave her dizzy and breathless. But a new electrical stimulation device is helping. Electrodes are attached to Henry's leg, then the leg piece is wrapped around. A wireless handheld device sends mini shocks through the leg muscles.

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If one had foot drop, then the one's knee is weak anyway. So if Henry said "my knee is weak, because I have what they call foot drop," I will get her well. But she used "but" there...

What on earth the but means there?

For example:

My mouth is very dry, because I in the house have no water to drink.

Can you say " my mouth is very dry, but I in the house have no water to drink?"
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"But" can be used in a lot of different ways. I've tried to come up with an explanation, but I realize explaining the usage of "but" is not easy at all. I am not an expert on knees or anything, but I can only say that in the first example, "My knee is weak, but I have...", I think that "but" is used to give more importance to the second part of the sentence. The weak knee is not the point, I am worried about the foot drop.
In the second example "but" is used to show that there's something wrong with the preceding statement: "My mouth is dry (so I'd like to drink something), but... (here's the problem)... but I have nothing to drink."
Just my opinion, anyway. Emotion: smile
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I think Kooyeen's explanation is right on.

As a reply to an obstinate orthopedic physician, I'd use, "I know my knee is weak. But I have etc." (Been there. Done that.)
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Comments  
You said: "If one had foot drop, then the one's knee is weak anyway"

That is not correct and that's what is creating problems for your understanding.

Drop foot is not the same as a weak knee.

His meaning is:

My knee is weak, (and in addition to that) I have what they call foot drop," Henry explains.
Thanks.

Native English speakers reconfirm this please.
 Kooyeen's reply was promoted to an answer.
Students: Are you brave enough to let our tutors analyse your pronunciation?
 Avangi's reply was promoted to an answer.
Thank you both.

Kooyeen, why is your English so good since you are, if I guess it rightly, an Italian?