In To Kill a Mockingbrid, Scout acquires an important realization during Tom’s testimony. What is it?

In Harper Lee's To Kill a
Mockingbird, Scout realizes several things during Tom Robinson's
trial.

First of all, she realizes that Atticus was
appointed to be Tom's lawyer, and he had to do it whether he wanted to or not, though I
don't think it would have occurred to him not to. He observes later that every lawyer
has a case that has a profound effect on him, and he expected this was the one for
him.

When Atticus has Heck Tate testify to Mayella Ewell's
injuries, we find that they are all on the right side of her face. This would mean that
she was struck by someone left-handed. Scout understand this, and understands the
implications of this information when Atticus exposes to the jury at Bob Ewell is
left-handed, but Scout doesn't think this means anything because Tom Robinson could also
be left-handed.

Scout soon realizes that Tom could
not have done the damage to Miss Mayella as she and her father
claimed. Tom's left arm had been caught in a cotton gin, and he could not use it: it was
difficult to keep in on the Bible when he was sworn
in.

Scout also comes to a realization about
Mayella.

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'Would Miss Mayella talk to
you?'

'Yes, sir, she talked to
me.'

As Tom Robinson gave his testimony, it came to me that
Mayella Ewell must have been the loneliest person in the world. She was even lonelier
than Boo Radley, who had not been out of the house in twenty-five years. When Atticus
asked if she had any friends, she seemed not to know what he meant, then she thought he
was making fun of her. She was as sad, I thought, as what Jem called a mixed child:
white people wouldn't have anything to do with her because she lived among pigs; Negroes
wouldn't have anything to do with her because she was white...Tom Robinson was probably
the only person who was ever decent to her, and when she stood up she looked at him as
if he were dirt beneath her
feet.

So Scout realizes
several things during the trial. She realizes that Tom could not have attacked Mayella
because when he gets up to be sworn in, his arm is totally useless, falling limply at
his side so he cannot keep it on the Bible's cover.

After
Tom begins to provide his testimony, Scout also realizes what a sad and lonely person
Mayella is.

Finally, and ironically, Scout notes that Tom
has probably been nicer to Mayella than anyone else and yet she treated him like a
criminal: and he is not.