Question

the boy is in geometry class. the boy is draw some lines about the blackboard. the lines are perpendicular.

there are two errors in this sentence.
the boy is draw some lines about the blackboard.

rewrite the sentence correctly.

Explanation:

The verb "draw" should be in the -ing form to be used with "is" for present - continuous tense. Also, "about" is the wrong preposition; it should be "on".

Answer:

The boy is drawing some lines on the blackboard.