I thought I was strong enough to do this, but I am/was not.

[northpath]

In a movie, one of the characters says:

I thought I was strong enough to do this, but I am not.

I think the right sentence should be:

I thought I was strong enough to do this, I was not

 

[5jj]

I thought I was strong enough to do this, but I am not. They are failing to do it.

I thought I was strong enough to do this, I was not. They failed to do it.

 

[Seve14]

I would say both can be correct. The first one conveys the idea the speaker still doesn't feel strong enough, so his being not strong enough refers to the present. In the second sentence the speaker didn't feel strong enough , so his not being strong enough refers to the past.

 

[northpath]

OK, what about the sentence:
I didn’t know you have a brother. (His/her brother is still alive!)

 

[emsr2d2]

OK, what about the sentence:
I didn’t know you have a brother. (His/her brother is still alive!)

It's understandable but the majority of native speakers would say "I didn't know you had a brother".

 

[Barque]

OK, what about the sentence:
I didn’t know you have a brother. (His/her brother is still alive!)

You could use "have" with a slightly different structure.

A: I'm going to be busy for the next few days. My brother and his family are visiting us.
B: You have a brother? I didn't know. For some reason I thought you were an only child. ("Had" wouldn't work here.)