‘有个男孩在教室里’的英语有个男孩在教室里 英语怎么说?在教室里有个男孩 英语怎么说?

There is a boy in the classroom.
It is the classroom that a boy is in.(强调句)或者In the classroom there is a boy.

there is a boy in the classroom.

you can't say "a boy is in the classroon" because it means 教室里的男孩. This is a phrase rather than a sentence.
有个男孩在教室里, you can say "there's a boy in the classroom"

A boy is in the classroon
There's a boy in the classroom

there is a boy in the classroom.

A boy is in the classroon
There's a boy in the classroom

A boy is in the classroon 有个男孩在教室里
There's a boy in the classroom 在教室里有个男孩

两句话都可以说是there is boy in the classroom