How do you think the tet offensive affect the morale of north vietnam?

It increased the morale drastically. It was a series of offensives by the Northern army on Southern soil and although in the end the US and the Southerners managed to win, they did so with heavy casualties. This showed that the US army was not invincible and rose the morale of the Northern troops drastically with it being a huge strategic victory due to propaganda that ensued.