1
I met a girl that she was born in Mongolia.
I met a girl who was born in Mongolia.
I met a girl that was born in Mongolia.
Here, who or that is the subject of was born. Do not add she as a second subject.
2
His car, that was very old, broke down on his way home.
His car, which was very old, broke down on his way home.
Use which, not that, to add extra information about a thing between commas.
In a relative clause without commas, we can use either word: ‘The car that broke down was very old.’ ‘The car which broke down was very old.’
3
I am taller that my brother.
I am taller than my brother.
Use than after a comparative: ‘taller than my brother’, ‘more expensive than my car’.
See THAN
4
Families are not as big nowadays that they were in the past.
Families are not as big nowadays as they were in the past.
To say that two things are similar or different in degree, use as + adjective + as: ‘My room is as big as yours.’ ‘Families are not as big as they were in the past.’
5
This is that people think nowadays.
This is what people think nowadays.
Use what to mean ‘the thing that’ or ‘the things that’: ‘Tell me what you think.’
See WHAT